feat(plugin-blocker): add blockerPlugin and useBlocker hook (#682)

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: ENvironmentSet

.changeset/add-plugin-blocker.md

@@ -0,0 +1,13 @@
+---
+"@stackflow/plugin-blocker": minor
+---
+
+Add blockerPlugin and useBlocker hook for navigation blocking
+
+- `useBlocker({ shouldBlock, onBlocked })` hook to declare blocking policies per activity
+- Blocking set model: multiple blockers can block a navigation, all must `proceed()` for it to execute
+- Only blockers from active (`isActive: true`) activities are evaluated
+- `onBlocked` notifications are serialized in navigation occurrence order (no re-entrancy)
+- Error isolation: one blocker's `onBlocked` throwing does not prevent other blockers from being notified
+- Plugin-level `onError` option for custom error handling (defaults to `console.error`)
+- Blocker cleanup on component unmount; previously captured `proceed` remains callable