wip(trust): park agent's signed-manifest module + release-root key off main

Moved here so main stays clean for the v1.7.98 release. Contains the trust/
module (canonical.rs, did.rs, signed_doc.rs) + seed::derive_release_root_ed25519.
Not wired into the build yet. Continue this work on this branch.

core/archipelago/src/trust/did.rs

@@ -0,0 +1,56 @@
+//! `did:key` <-> Ed25519 public key, mirroring the encoding already used by
+//! `identity_manager` so release-root DIDs are interchangeable with node DIDs.
+//!
+//! Format: `did:key:z<base58btc(0xed01 || 32-byte-pubkey)>`
+//! (`0xed01` is the multicodec varint prefix for an Ed25519 public key.)
+
+use anyhow::{anyhow, Context, Result};
+use ed25519_dalek::VerifyingKey;
+
+const ED25519_MULTICODEC: [u8; 2] = [0xed, 0x01];
+
+/// Encode an Ed25519 public key as a `did:key` string.
+pub fn did_key_for_ed25519(key: &VerifyingKey) -> String {
+    let mut bytes = Vec::with_capacity(34);
+    bytes.extend_from_slice(&ED25519_MULTICODEC);
+    bytes.extend_from_slice(key.as_bytes());
+    format!("did:key:z{}", bs58::encode(bytes).into_string())
+}
+
+/// Decode a `did:key` string into an Ed25519 verifying key.
+pub fn ed25519_pubkey_from_did_key(did: &str) -> Result<VerifyingKey> {
+    let z_part = did
+        .strip_prefix("did:key:z")
+        .ok_or_else(|| anyhow!("invalid did:key format: {}", did))?;
+    let decoded = bs58::decode(z_part)
+        .into_vec()
+        .context("invalid base58 in did:key")?;
+    if decoded.len() != 34 || decoded[0..2] != ED25519_MULTICODEC {
+        return Err(anyhow!("not an Ed25519 did:key (bad multicodec prefix)"));
+    }
+    let arr: [u8; 32] = decoded[2..]
+        .try_into()
+        .expect("length checked above");
+    VerifyingKey::from_bytes(&arr).context("invalid Ed25519 public key in did:key")
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use ed25519_dalek::SigningKey;
+
+    #[test]
+    fn roundtrip() {
+        let key = SigningKey::from_bytes(&[3u8; 32]).verifying_key();
+        let did = did_key_for_ed25519(&key);
+        assert!(did.starts_with("did:key:z6Mk"), "got {}", did);
+        let back = ed25519_pubkey_from_did_key(&did).unwrap();
+        assert_eq!(key.as_bytes(), back.as_bytes());
+    }
+
+    #[test]
+    fn rejects_non_ed25519() {
+        assert!(ed25519_pubkey_from_did_key("did:key:zQ3shazz").is_err());
+        assert!(ed25519_pubkey_from_did_key("not-a-did").is_err());
+    }
+}