feat(dom): per-root awaitCommit / commitGeneration filtering (app-scoped superset today)

[stefvw93]

Summary

Follow-up to the Loom render scheduler (PR #170 / v0.31.0). RootHandle.awaitCommit and RootHandle.commitGeneration are app-scoped: one Loom serves all roots of a WeftApp, so with multiple roots awaitCommit may wait on sibling roots' in-flight work. This is documented as a deliberate superset (may over-wait), not a bug (LM11, loom.specs.md:129).

Proposal

Add per-root filtering so RootHandle.awaitCommit resolves once everything dirty for that root at call time has committed or been discarded, independent of sibling roots:

• Tag each registered cell with its owning root (cells already carry a scope; a root id can be threaded through LoomRegisterOptions or derived from the render context).
• awaitCommit barriers filter on the root tag; commitGeneration optionally exposes a per-root generation alongside the app-scoped one.

Constraints

• Single-root apps (the common case) behave identically.
• The app-scoped commitGeneration stays available (some callers want global quiescence).
• No change to the flush loop's single-fiber, ascending-seq commit ordering.

References

• LM11 (RootHandle surface, app-scoped superset) — packages/dom/src/client/loom.specs.md:129
• awaitCommit quiescence scope note — loom.specs.md:304
• Listed as out-of-scope follow-up in loom.specs.md (line 32).