[rushd][WS0] Upstream engine prerequisites (in rush-lib)

Start with dependency #5378 and the committed rush-lib.api.md to understand the stateful IOperationGraph and OperationGraphHooks APIs, then review the listed in-repo plugins and the unchecked acceptance criteria. Done means per-iteration runner lifetime behavior is implemented and validated without regressions; the operation-graph library convergence and migration documentation are follow-up items.

Land the watch-mode overhaul and adapt in-repo plugins so the engine exposes a stateful, warm-able operation graph the daemon can own.

Depends on: #5378 (provides the stateful IOperationGraph)

Scope

• Merge the watch-mode overhaul (#5378). Land the stateful IOperationGraph (setEnabledStates, scheduleIterationAsync, executeScheduledIterationAsync, invalidateOperations, abortCurrentIterationAsync, closeRunnersAsync), OperationGraphHooks (onIdle, onExecutionStatesUpdated), IPCOperationRunner, and the reworked ProjectWatcher — replacing the disposable OperationExecutionManager with a graph that persists across iterations.
• Migrate in-repo plugins. Move CacheableOperationPlugin, ShardedPhaseOperationPlugin, OperationResultSummarizerPlugin, ConsoleTimelinePlugin, PhasedOperationPlugin, IPCOperationRunnerPlugin, rush-serve-plugin, rush-bridge-cache-plugin, and rush-buildxl-graph-plugin onto the new hooks.
• One warm full-workspace graph. Build the graph over all workspace projects with a client's selection applied as an enabled subset (includeAllProjectsInWatchGraph) so the daemon keeps a single warm graph and scopes it per request.
• Per-iteration runner lifetime. Lift the persistent-vs-one-shot choice out of static IPCOperationRunnerPlugin construction into a per-iteration host decision — keep hot runners resident for warm projects; tear cold ones down immediately at operation completion.
• Follow-up (non-blocking). Converge rush-lib's OperationGraph onto the standalone @rushstack/operation-graph library.

Acceptance criteria

• The stateful IOperationGraph and OperationGraphHooks are merged and exported, with the public API captured in the committed rush-lib.api.md.
• All existing phased-command and watch-mode behavior is preserved; updated snapshots are intentional and reviewed.
• Every in-repo plugin compiles and passes its tests against OperationGraphHooks (no plugin depends on a removed/renamed hook except through the optional compat shim in WS5); cache, sharding, summary, and timeline outputs are unchanged.
• A full-workspace graph can be built once and an arbitrary selection enabled via setEnabledStates without rebuilding; disabled projects stay resident (watchable/invalidatable) but unscheduled.
• The host can choose persist vs one-shot per operation per iteration (default unchanged when unspecified); a non-persistent runner is torn down immediately at its operation's completion, before downstream operations run.
• (Follow-up, not on the critical path) rush-lib consumes @rushstack/operation-graph with no behavioral change.
• Breaking hook renames/relocations are recorded in the WS5 migration guide + API changelog; new engine behavior ships opt-in / dead-code until cutover; unit/integration tests green in CI.

Part of #5894.