fix(mesh): native Meshtastic unicast DMs + driver-level E2E status

Meshtastic DMs were falling back to a channel broadcast, so every node
on the LoRa channel saw a "direct" message. Send a directed MeshPacket
(to = node num, decoded from the synthetic pubkey's node-id bytes)
instead — the Meshtastic analog of the meshcore CMD_SEND_TXT_MSG fix.
DMs now reach only the recipient; firmware auto-PKC-encrypts them
end-to-end once NodeInfo keys are exchanged.

Capture E2E status at the driver level (no shared-type/UI change):
- learn each peer's real Curve25519 key from User.public_key (field 8)
  and inbound MeshPacket.public_key (16), kept in a side-map separate
  from the synthetic routing key so unicast routing is untouched
- detect inbound MeshPacket.pki_encrypted (17) to tell a true E2E DM
  from a channel-PSK fallback
- peer_is_pkc_capable() seam for a future mesh-tab E2E badge

Hot-swap preserved: no dispatched MeshRadioDevice signature or the
shared ParsedContact changed, so meshcore and meshtastic stay
interchangeable behind the listener.

Adds tests/multinode/meshtastic.sh, a two/three-radio on-air parity
harness (detect, discover, DM round-trip, DM privacy, channel
broadcast, typed envelope, reachability).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

core/archipelago/src/mesh/meshtastic.rs

@@ -42,6 +42,14 @@ pub struct MeshtasticDevice {
     long_name: Option<String>,
     short_name: Option<String>,
     contacts: HashMap<u32, ParsedContact>,
+    /// Real Curve25519 public keys, keyed by node-num, as learned from NodeInfo
+    /// (`User.public_key`) or PKC-encrypted inbound packets (`MeshPacket
+    /// .public_key`). Kept SEPARATE from `contacts[*].public_key_hex`, which is
+    /// the synthetic node-num-derived routing key that `send_text_msg` relies
+    /// on — we must not overwrite that or unicast routing breaks. This map only
+    /// records which peers are PKC-capable, so we can tell a true end-to-end
+    /// (PKI) DM from a channel-PSK fallback.
+    peer_pubkeys: HashMap<u32, Vec<u8>>,
     device_path: String,
 }
 
@@ -68,6 +76,7 @@ impl MeshtasticDevice {
             long_name: None,
             short_name: None,
             contacts: HashMap::new(),
+            peer_pubkeys: HashMap::new(),
             device_path: path.to_string(),
         })
     }
@@ -150,12 +159,32 @@ impl MeshtasticDevice {
             .await
     }
 
-    /// Meshtastic addresses by numeric node-id, not a meshcore pubkey prefix,
-    /// so there's no direct unicast mapping here. Best-effort fallback to a
-    /// channel send keeps the device interface uniform; native unicast is only
-    /// meaningful on the Meshcore transport.
-    pub async fn send_text_msg(&mut self, _dest_pubkey_prefix: &[u8; 6], msg: &[u8]) -> Result<()> {
-        self.send_channel_text(0, msg).await
+    /// Native Meshtastic unicast DM. Our synthetic Meshtastic pubkeys carry the
+    /// numeric node-id in their first 4 bytes (little-endian, see
+    /// `synthetic_pubkey`), so `dest_pubkey_prefix` directly yields the
+    /// destination node number. We send a directed MeshPacket (`to` = node num)
+    /// rather than a `BROADCAST_NUM` channel blast — this is the Meshtastic
+    /// analog of the meshcore `CMD_SEND_TXT_MSG` fix: the message is delivered
+    /// as a real DM (only the recipient's client surfaces it) instead of
+    /// polluting the shared primary channel where every node would see it.
+    ///
+    /// If the prefix decodes to node 0 / broadcast (e.g. a non-Meshtastic
+    /// synthetic key routed here by mistake), fall back to a channel send so the
+    /// device interface stays uniform and the message still goes out.
+    pub async fn send_text_msg(&mut self, dest_pubkey_prefix: &[u8; 6], msg: &[u8]) -> Result<()> {
+        let node_num = u32::from_le_bytes([
+            dest_pubkey_prefix[0],
+            dest_pubkey_prefix[1],
+            dest_pubkey_prefix[2],
+            dest_pubkey_prefix[3],
+        ]);
+        if node_num == 0 || node_num == BROADCAST_NUM {
+            return self.send_channel_text(0, msg).await;
+        }
+        let text = String::from_utf8_lossy(msg);
+        let packet = encode_mesh_packet(node_num, TEXT_MESSAGE_APP, text.as_bytes());
+        self.send_to_radio(&encode_to_radio_variant(TO_RADIO_PACKET, &packet))
+            .await
     }
 
     /// Meshtastic has no meshcore-style contact table; these are no-ops so the
@@ -214,6 +243,19 @@ impl MeshtasticDevice {
         Ok(self.handle_from_radio(&frame))
     }
 
+    /// Whether we've learned `node_num`'s real PKI (Curve25519) key — from a
+    /// NodeInfo `public_key` or an inbound PKC DM — meaning the firmware can
+    /// deliver DMs to/from it end-to-end encrypted instead of falling back to
+    /// the channel PSK. Driver-internal for now; lets a future mesh-tab badge
+    /// distinguish a true E2E DM from a channel-encrypted one without changing
+    /// the shared device interface (which would break meshcore hot-swap).
+    #[allow(dead_code)] // seam: consumed when the mesh-tab E2E badge lands
+    pub fn peer_is_pkc_capable(&self, node_num: u32) -> bool {
+        self.peer_pubkeys
+            .get(&node_num)
+            .is_some_and(|k| !k.is_empty())
+    }
+
     pub fn advert_name(&self) -> Option<String> {
         self.long_name
             .clone()
@@ -286,6 +328,15 @@ impl MeshtasticDevice {
 
     fn update_node_info(&mut self, data: &[u8]) {
         if let Some(node) = parse_node_info(data) {
+            if let Some(pk) = node.public_key.as_ref() {
+                if self.peer_pubkeys.insert(node.num, pk.clone()).is_none() {
+                    debug!(
+                        node = node.num,
+                        key_len = pk.len(),
+                        "Meshtastic peer is PKC-capable (NodeInfo public_key)"
+                    );
+                }
+            }
             let key = synthetic_pubkey(node.num);
             let name = node
                 .long_name
@@ -318,6 +369,18 @@ impl MeshtasticDevice {
         if Some(from) == self.node_num {
             return None;
         }
+        // Record E2E status: a `pki_encrypted` packet (or one carrying the
+        // sender's `public_key`) proves this DM arrived end-to-end encrypted via
+        // the PKI, not the shared channel PSK. We learn the sender's key here too
+        // — but keep it OUT of the routing `public_key_hex` (synthetic) so the
+        // device interface stays identical to meshcore's and the two remain
+        // hot-swappable behind the mesh listener.
+        if let Some(pk) = packet.public_key.as_ref() {
+            self.peer_pubkeys.entry(from).or_insert_with(|| pk.clone());
+        }
+        if packet.pki_encrypted {
+            debug!(node = from, "Meshtastic DM received end-to-end encrypted (PKI)");
+        }
         let from_key = synthetic_pubkey(from);
         self.contacts.entry(from).or_insert_with(|| ParsedContact {
             public_key_hex: hex::encode(synthetic_pubkey(from)),
@@ -444,6 +507,7 @@ struct ParsedNode {
     long_name: Option<String>,
     short_name: Option<String>,
     last_heard: Option<u32>,
+    public_key: Option<Vec<u8>>,
 }
 
 fn parse_node_info(data: &[u8]) -> Option<ParsedNode> {
@@ -454,6 +518,7 @@ fn parse_node_info(data: &[u8]) -> Option<ParsedNode> {
         long_name: None,
         short_name: None,
         last_heard: None,
+        public_key: None,
     };
     while idx < data.len() {
         let (field, value, next) = next_field(data, idx)?;
@@ -466,6 +531,7 @@ fn parse_node_info(data: &[u8]) -> Option<ParsedNode> {
                     node.id = user.id;
                     node.long_name = user.long_name;
                     node.short_name = user.short_name;
+                    node.public_key = user.public_key;
                 }
             }
             (5, FieldValue::Fixed32(v)) => node.last_heard = Some(v),
@@ -483,6 +549,7 @@ struct ParsedUser {
     id: Option<String>,
     long_name: Option<String>,
     short_name: Option<String>,
+    public_key: Option<Vec<u8>>,
 }
 
 fn parse_user(data: &[u8]) -> Option<ParsedUser> {
@@ -491,6 +558,7 @@ fn parse_user(data: &[u8]) -> Option<ParsedUser> {
         id: None,
         long_name: None,
         short_name: None,
+        public_key: None,
     };
     while idx < data.len() {
         let (field, value, next) = next_field(data, idx)?;
@@ -499,6 +567,9 @@ fn parse_user(data: &[u8]) -> Option<ParsedUser> {
             (1, FieldValue::Bytes(b)) => user.id = string_field(b),
             (2, FieldValue::Bytes(b)) => user.long_name = string_field(b),
             (3, FieldValue::Bytes(b)) => user.short_name = string_field(b),
+            // User.public_key (field 8): the peer's Curve25519 key. Its presence
+            // means the radio can PKC-encrypt DMs to this node end-to-end.
+            (8, FieldValue::Bytes(b)) if !b.is_empty() => user.public_key = Some(b.to_vec()),
             _ => {}
         }
     }
@@ -509,18 +580,28 @@ struct ParsedPacket {
     from: Option<u32>,
     portnum: u32,
     payload: Vec<u8>,
+    /// MeshPacket.pki_encrypted (field 17): the firmware decrypted this packet
+    /// with the PKI (Curve25519) key, i.e. it arrived end-to-end encrypted
+    /// rather than via the shared channel PSK.
+    pki_encrypted: bool,
+    /// MeshPacket.public_key (field 16): the sender's key, carried on PKC DMs.
+    public_key: Option<Vec<u8>>,
 }
 
 fn parse_mesh_packet(data: &[u8]) -> Option<ParsedPacket> {
     let mut idx = 0;
     let mut from = None;
     let mut decoded = None;
+    let mut pki_encrypted = false;
+    let mut public_key = None;
     while idx < data.len() {
         let (field, value, next) = next_field(data, idx)?;
         idx = next;
         match (field, value) {
             (1, FieldValue::Fixed32(v)) => from = Some(v),
             (4, FieldValue::Bytes(b)) => decoded = Some(b),
+            (16, FieldValue::Bytes(b)) if !b.is_empty() => public_key = Some(b.to_vec()),
+            (17, FieldValue::Varint(v)) => pki_encrypted = v != 0,
             _ => {}
         }
     }
@@ -541,6 +622,8 @@ fn parse_mesh_packet(data: &[u8]) -> Option<ParsedPacket> {
         from,
         portnum,
         payload,
+        pki_encrypted,
+        public_key,
     })
 }