feat(homecore-plugins/p2): Wasmtime runtime + example WASM plugin (resolves ADR-128 Q2)

- Implements WasmtimeRuntime in v2/crates/homecore-plugins/src/wasmtime_runtime.rs
  with a Wasmtime 25 Cranelift JIT engine. Registers 4 host imports via Linker:
  hc_state_get, hc_state_set, hc_state_subscribe, hc_log. Each plugin gets an
  isolated Store<PluginStoreData> holding a HomeCore handle + subscription list.

- Adds host_abi.rs documenting the JSON-over-linear-memory wire format (public
  ABI spec for plugin authors). Max buffer 64 KiB. ConfigEntryJson and
  StateChangedEventJson are the canonical wire types.

- Creates v2/crates/homecore-plugin-example/ (wasm32-unknown-unknown, excluded
  from workspace per wifi-densepose-wasm-edge pattern). The plugin monitors
  sensor.test_temp and sets binary_sensor.test_alert on/off at 25/20 thresholds.

- Adds tests/integration.rs with 3 tests: compiled .wasm end-to-end round-trip,
  WAT-based fallback (always runs), and linker smoke test. All 15 tests pass
  (12 unit + 3 integration) under --features wasmtime.

- ADR-128 Q2 resolved: Wasmtime is the chosen runtime for P2. WASM3 stays as
  future fallback under --features wasm3 for constrained hardware (ADR-128 §8).

Co-Authored-By: claude-flow <ruv@ruv.net>

v2/Cargo.lock

@@ -3528,8 +3528,10 @@ dependencies = [
  "serde_json",
  "thiserror 1.0.69",
  "tokio",
+ "uuid",
  "wasm3",
  "wasmtime",
+ "wat",
 ]
 
 [[package]]
@@ -3539,8 +3541,10 @@ dependencies = [
  "async-trait",
  "chrono",
  "homecore",
+ "ruvector-core",
  "serde",
  "serde_json",
+ "sha2",
  "sqlx",
  "thiserror 1.0.69",
  "tokio",