Add disposeFiber() function that performs bottom-up teardown of fiber
subtrees: recursively disposes children before parents, calls
host.finalizeInstance for per-instance cleanup, invokes signal disposers,
and clears fiber state. Idempotent via disposed flag. Does NOT call
host.removeChild (that's the commit phase's job).
- Add disposeFiber + HostLike to src/host/fiber.ts
- Add finalizeInstance to HostConfig interface
- Add disposed boolean to Fiber interface
- Export disposeFiber and HostLike from barrel
- Add 7 tests for disposeFiber (3-level tree, idempotency, signal cleanup, etc.)
Adds commitMutations function to reconcile.ts that processes fiber effects
in correct order: removes (reverse), inserts+moves (left-to-right with
insertBefore), updates (top-down). Integrates key-based reconciliation
pipeline into render() via reconcileNode, resolveUNode, and
createFiberForInsert.
