Add bounded engine-thread action queue

[EricSun0218]

What changed

• add an engine-neutral QueuedGameActionHandler over the existing IGameActionHandler contract
• provide bounded FIFO admission, per-pump work limits, active-operation bounds, queued cancellation, stop/dispose lifecycle, and single-thread pump ownership
• route both execute and recovery callbacks through the host-owned engine thread
• preserve the existing durable journal and receipt protocol; started mutations are never canceled solely because the original caller timed out
• document the integration pattern and expose it in the capability tables

Why

In-process Unity, Godot, and other C# hosts currently have to rebuild the same background-to-engine-thread action handoff. The reusable queue removes that boilerplate without moving game authority or generation validation into the framework.

Validation

• complete Release solution build: 0 warnings, 0 errors
• complete Release suite: 885 tests passed
• queue concurrency/lifecycle suite repeated 10/10 times
• API surface freeze and formatting checks passed
• public-tree validation passed
• Godot 4.7.1 .NET real-editor smoke passed
• Unity 6000.5.6f1 real-editor smoke passed

Fixes #33