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Add ParameterEventHandler for cross-node parameter monitoring (#1438)

Adds `ParameterEventHandler`, a new class that monitors parameter changes on any node in the ROS 2 graph by subscribing to the `/parameter_events` topic. This is the rclnodejs equivalent of rclpy's `ParameterEventHandler` and supports two callback types: per-parameter callbacks (filtered by parameter name and node name) and event-level callbacks (for every `ParameterEvent` message). Refactors `ParameterWatcher` to use `ParameterEventHandler` internally, eliminating its direct `/parameter_events` subscription while preserving full API compatibility.

**New files:**

- `lib/parameter_event_handler.js` — Core implementation with `addParameterCallback()`, `removeParameterCallback()`, `addParameterEventCallback()`, `removeParameterEventCallback()`, `destroy()`, and static helpers `getParameterFromEvent()` / `getParametersFromEvent()`. Uses opaque handle objects for add/remove symmetry matching rclpy. Callbacks dispatched in FILO order. QoS option correctly forwarded via options object to `createSubscription()`. Parameter names trimmed before storage to prevent whitespace mismatch. Dispatch loop inverted to O(event_params) via direct Map lookups instead of O(registered_callbacks).
- `types/parameter_event_handler.d.ts` — Full TypeScript declarations for `ParameterEventHandler`, `ParameterCallbackHandle`, `ParameterEventCallbackHandle`, and `ParameterEventHandlerOptions`.
- `test/test-parameter-event-handler.js` — 23 tests covering constructor validation, parameter callbacks (add/remove/filtering by node and param name), event callbacks, static helpers, lifecycle/destroy, and Node convenience method integration.

**Modified files:**

- `lib/node.js` — Added `createParameterEventHandler()` and `destroyParameterEventHandler()` convenience methods with lifecycle tracking via `_parameterEventHandlers` array. Auto-cleanup on node destruction.
- `index.js` — Exported `ParameterEventHandler` cla... (continued)

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