feat: add observability module and wire MeshNode run() with background tasks

Add health checks (/healthz), Prometheus metrics export (/metricsz),
and tracing spans to the P2P mesh node. MeshNode.run() starts GC and
health server as background tasks, returning a RunHandle for lifecycle
management. Health endpoint returns 503 during graceful shutdown drain.

docs/status.md

@@ -1,5 +1,41 @@
 # Status Log
 
+## 2026-04-11 — Observability & MeshNode run() wiring
+
+### Completed
+- **observability.rs** — new module with health checks, Prometheus text export, HTTP server
+  - `NodeHealth` struct with per-subsystem health checks (transport, routing, store)
+  - `HealthStatus` enum (Healthy/Degraded/Draining/Unhealthy) with HTTP status codes
+  - `prometheus_text()` — renders `MetricsSnapshot` in Prometheus exposition format
+  - `HealthServer` — lightweight TCP-based HTTP server for `/healthz` and `/metricsz`
+- **MeshNode.run()** — starts background tasks and returns a `RunHandle`
+  - Periodic GC task (store, routing table, rate limiters) with configurable interval
+  - Health/metrics HTTP server (optional, via `MeshNodeBuilder.health_listen()`)
+  - Shutdown coordination via `watch` channel
+- **RunHandle** — public API for interacting with a running node
+  - `.node()` — access to the MeshNode
+  - `.health()` — current health snapshot
+  - `.metrics_snapshot()` — current metrics
+  - `.health_addr()` — bound health server address
+  - `.shutdown()` — graceful shutdown (signals tasks + drains transports)
+- **Tracing spans** — `#[tracing::instrument]` on `process_incoming()` and `send()`
+  - Includes sender/dest address and payload length as span fields
+  - GC cycle wrapped in `mesh_gc` info span
+- **Draining flag** — `AtomicBool` for shutdown awareness; health endpoint returns 503
+
+### Test Coverage
+- 232 total tests passing (212 lib + 3 fapp_flow + 1 meshservice + 16 multi_node)
+- 7 new observability unit tests (health healthy/degraded/draining, prometheus format)
+- Full workspace `cargo check` clean
+
+### What's Next
+1. Wire `MeshNode.run()` into an example binary or the server
+2. Announce loop task (periodic re-announce to neighbors)
+3. Grafana dashboard for mesh metrics
+4. Integration test for health HTTP endpoint
+
+---
+
 ## 2026-04-01 — meshservice workspace integration
 
 ### Completed