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.
First render still does a full mount and builds fiber tree. Subsequent
renders reconcile the new UNode tree against existing fibers: compare
props via prepareUpdate, apply changes via commitUpdate. Excess old
children remain (structural changes deferred to Phase 2). Function
components remain transparent during reconciliation.
