feat(executor): cooperative single-thread lanes + generic config-driven lane routing

[rlch]

Two related changes to the #[frb(thread = ...)] executor-routing feature.

1. Cooperative single-thread lanes (fix)

ThreadExecutor::execute_async drove each async task with futures::executor::block_on on the lane's single worker, so any .await inside a #[frb(thread = ...)] handler parked that worker until the future resolved — every other call on the same lane queued behind it. This is a native-only regression vs SimpleExecutor (spawns onto a multi-thread runtime) and the wasm lane (spawn_locals, yielding).

• SingleThreadAsyncLane (native thread_pool/io.rs): one dedicated thread running a current_thread runtime + LocalSet, jobs run inside the LocalSet.
• Native spawn_local_compat now tokio::task::spawn_locals the task future instead of block_on-ing it, so .await yields the lane while keeping single-thread FIFO ownership — matching wasm.

Regression test single_worker_lane_pinning_tests: two async tasks on one single-worker lane with a cross-task dependency — deadlocks under block_on, passes under cooperative scheduling.

2. Generic, config-driven lane routing (feat)

The feature baked a consumer's domain into FRB: Thread hardcoded Loro/Export, the attribute parser allowlisted those names, and validate_frb_threads hardcoded session_id/drive_id/… param names + a "sync or session-handle ⇒ must annotate" rule. FRB should know none of it.

• Thread is now { Main, Worker(&'static str) } — the lane key is opaque; the embedder's ThreadRouter resolves it.
• Attribute parser accepts any ident; codegen emits Thread::Main / Thread::Worker("<ident>").
• New optional lane_routing config (flutter_rust_bridge.yaml): lanes (valid keys) + require_annotation_when { sync, param_named }. validate_frb_threads enforces it against config; absent config ⇒ no-op, so FRB ships with zero domain knowledge.

lane_routing:
  lanes: [Loro, Export]
  require_annotation_when:
    sync: true
    param_named: [session_id, drive_id, browser_id, wizard_id]

Tests

frb_rust 37/37; frb_codegen attribute + config-golden tests green. WASM unchanged. (Pre-existing oxidized MIR-golden drift that predates this branch is left untouched — unrelated to this change.)

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