feat(orchestrator): Phase 3.2 — wire Quadlet path behind feature flag

prod_orchestrator::install_fresh now branches on the new
Config::use_quadlet_backends flag (default false):

* off (today's production behavior) — unchanged: runtime.create_container
  + start_container, container parented under archipelago.service's
  cgroup, FM3 cascade SIGKILL on every archipelago restart.
* on  — install_via_quadlet renders the manifest as a Quadlet unit via
  QuadletUnit::from_manifest, writes it atomically into
  ~/.config/containers/systemd/, calls daemon-reload, and starts the
  generated <name>.service. Container ends up under user.slice — no
  more cgroup parented under archipelago, so archipelago restarts
  don't touch the container's lifetime.

Default off so this commit is structurally safe to ship: nothing
changes at runtime until an operator opts in. Flip the default once
tests/lifecycle/run-20x.sh has gone green against the new path on
.228 + .198 (the v1.7.52 release gate).

Plumbing:
* config.rs — `use_quadlet_backends: bool` w/ Default false
* prod_orchestrator.rs — flag stored on the struct, threaded through
  new(), with set_use_quadlet_backends(bool) test setter
* prod_orchestrator.rs — install_via_quadlet helper
* dropped the Phase-3.1 #[allow(dead_code)] markers on from_manifest /
  parse_memory_mib / RestartPolicy::OnFailure now that the call path
  exists; if a future revert removes the wiring, the warnings come back.

Tests: 624 passing, cargo check clean (0 warnings). Existing companion
behavior unaffected — render_skips_backend_directives_when_default
still passes byte-equal to before quadlet.rs grew the new fields.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

core/archipelago/src/config.rs

@@ -62,6 +62,14 @@ pub struct Config {
     /// Tor SOCKS5 proxy (e.g. 127.0.0.1:9050). When set, ALL Nostr traffic routes through Tor.
     #[serde(default)]
     pub nostr_tor_proxy: Option<String>,
+    /// Phase 3.2 of v1.7.52: route orchestrator-managed backend installs
+    /// through Quadlet (`.container` units in ~/.config/containers/systemd
+    /// + systemctl --user start) instead of `podman create + start`. Default
+    /// off so the legacy path stays the production path until the harness
+    /// at tests/lifecycle/run-20x.sh has gone green against the new path
+    /// on .228 + .198. See `project_v1_7_52_phase3_quadlet_design`.
+    #[serde(default)]
+    pub use_quadlet_backends: bool,
 }
 
 impl Config {
@@ -221,6 +229,7 @@ impl Default for Config {
                 "wss://relay.nostr.info".into(),
             ],
             nostr_tor_proxy: Some("127.0.0.1:9050".into()),
+            use_quadlet_backends: false,
         }
     }
 }