host/observer: phase 3 pure core — RegistryModel reducer + classifiers

My half of the phase-3 split (impl plan §4). Pure, no PipeWire: the adapter
(Codex's half) translates live registry callbacks / binds / /proc reads /
core.sync into RegEvents and feeds this reducer.

- RegistryModel::apply folds RegEvents into serial-keyed maps with an
  insertion-ordered id index so global_remove accounts for the oldest
  generation first; recycled ids stay Ambiguous until accounted (v3.4 §6.1.3).
- Readiness epoch: graph_ready false until ServerSynced + no outstanding
  obligations (withheld nodes, pending link binds); bounded timeout fails
  closed. Gates sticky retirement only; sticky once terminal.
- session_device classifier: hardware-PCM factory allowlist, exact match,
  fail closed to false; a node on an unresolved Device is withheld, never
  admitted provisional.
- pulse-PID derivation split into pure candidate (repeated sec_pid) + validate
  (/proc comm), so the 6-case failure matrix is unit-testable; any failure =>
  None (key 4 unusable).

34 tests cover 5 of 6 exit-gate rows (the live topology-diff row is the
adapter's). cargo test --bins 117 green, fmt + clippy clean.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/host/mod.rs

@@ -1,5 +1,6 @@
 pub mod audio;
 mod capture;
+mod observer;
 mod pipeline;
 mod quality;
 mod serve;