render: the M3 OFX host — one oak-worker --ofx-host process for every plugin job

docs/zh/plans/render-pipeline-threads.md M3 (design 3.2): OpenFX crash
isolation moves from "every worker hosts plugins" to a single dedicated
host process, served over NDJSON + shared memory.

- oak-worker --ofx-host mode (src/ofx_host.rs): loads every plugin once,
  resolves jobs by the cross-process-stable OFX identifier, and renders
  through the same in-process executor the workers used to install.
- oak-render/ofxhost.rs: the single-host client. The render manager
  creates and installs it for the Pipeline backend (lazy spawn on the
  first plugin job); eval::process_plugin_job prefers it and falls back
  to the in-process executor otherwise, so the process backend keeps its
  current behavior until M4.
- Data plane: input/output FrameSlotPool pairs (the handshake's input_*
  fields are used for the first time). Named clips and the source frame
  are written to input slots after the explicit CPU readback; the plugin
  output returns through an output slot. Pool size/capacity grow by a
  host restart when a job needs more (safe: submissions are serialized
  and one job is in flight).
- Crash loop: reader EOF fails the in-flight submit, which respawns the
  host and re-posts the same job (frames are read back once); after three
  consecutive crashes the client is permanently dead and the evaluator
  falls back to a purple frame. The dead child is reaped immediately, and
  a submit mutex enforces the one-job-in-flight contract.
- Progress/cancel: the host flushes plugin_progress immediately (live
  progress), and reads stdin on its own thread so plugin_cancel takes
  effect mid-render at the plugin's next progressUpdate; the sticky flag
  resets at progressStart and request_plugin_cancel_all broadcasts to
  both the worker pool and the host.
- JobSpec::Plugin / PluginJobPayload carry the plugin type_id (stable
  across processes); `--ofx-crash-once` / `--ofx-crash-always` are the
  deterministic crash hooks, matching the worker's env hooks.
- Tests: wire round-trips; host unit tests (crash budget, cancel-flag
  reset through the factory, source mapping); oak-worker integration
  tests against the real host + bundled test plugin (render + progress,
  crash respawn and re-post, three-crash give-up, mid-render cancel on
  the new slow variant, concurrent submits); eval's purple fallback.

crates/oak-node/src/nodes/plugin.rs

@@ -80,6 +80,9 @@ impl PluginInstanceHandle {
 pub struct PluginJobPayload {
 	/// The instance identity (oakplugin registry key).
 	pub instance: PluginInstanceHandle,
+	/// The OFX plugin identifier (cross-process stable; the single OFX
+	/// host process resolves its own instance from it).
+	pub type_id: String,
 	/// The request time (C++ `globals.time().in()`).
 	pub time: Rational,
 	/// The effect input id the main source texture arrives on (C++
@@ -238,6 +241,7 @@ impl NodeBehavior for PluginNode {
 		if tex.is_some() && !self.instance.is_null() {
 			let payload = Job::PluginJob(PluginJobPayload {
 				instance: self.instance,
+				type_id: self.type_id.clone(),
 				time,
 				effect_input_id: core.effect_input.clone(),
 				values: inputs.clone(),