feat(homecore-plugins/p1): ADR-128 plugin runtime scaffold

Adds `v2/crates/homecore-plugins` (0.1.0-alpha.0) — the P1 scaffold for
the HOMECORE-PLUGINS WASM integration system (ADR-128):

- `manifest.rs`: `PluginManifest` — superset of HA manifest.json; serde
  round-trip + required-field validation (`domain`/`name`/`version`).
- `error.rs`: `PluginError` typed enum (InvalidManifest, AlreadyLoaded,
  NotFound, RuntimeError, SetupFailed, UnloadFailed, Io).
- `plugin.rs`: `HomeCorePlugin` async trait + `PluginId` newtype.
- `runtime.rs`: `PluginRuntime` trait + `InProcessRuntime` (native Rust,
  first-party plugins). `WasmtimeRuntime` stub gated on `--features wasmtime`
  (default-off; 30 MB dep deferred to P2).
- `registry.rs`: `PluginRegistry<R>` — load/unload/list/contains via RwLock.
- 10 unit tests, 0 failed.

Wasmtime vs wasm3 runtime selection is still open (ADR-128 §8 Q2);
this scaffold makes the choice swappable via the `PluginRuntime` trait.
The `wasmtime` and `wasm3` features are default-off; P2 resolves the choice
and wires host ABI (`hc_state_get`/`hc_state_set`/etc.) to ADR-127.

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

v2/Cargo.toml

@@ -29,6 +29,8 @@ members = [
     "crates/nvsim",
     "crates/nvsim-server",
     "crates/homecore",                 # ADR-127 — HOMECORE state machine
+    "crates/homecore-plugins",         # ADR-128 — HOMECORE-PLUGINS WASM runtime (P1 scaffold)
+    "crates/homecore-api",             # ADR-130 — HOMECORE REST + WS API
     # ADR-100/ADR-101: Cognitum Cog packaging — first Cog from this repo.
     # Ships the wifi-densepose pose-estimation model as a signed binary +
     # JSONL manifest installable by the Cognitum V0 appliance (cognitum-v0,