fix(pi): clear watcher shutdown latch on session_start (/new)

[moj02090]

Summary

Fixes Pi /new leaving the primary watcher stuck on watcher: not armed - Pi session is shutting down.

Root cause

.pi/extensions/fm-primary-pi-watch.ts sets a module-scope stopping latch in stopArm() on session_shutdown / process exit. /new keeps the same Pi process and loaded extension module, so the old session_start path (only markLoaded()) never cleared that latch. Every later arm refused with the shutdown message.

Fix

On session_start:

1. Clear stopping = false
2. Reset retryFailures / retryTimer
3. Re-attach the process-exit cleanup listener
4. markLoaded() as before
5. Quiet startArm() when this session already owns the lock and nothing is armed

Review follow-ups on the same branch also:

• Cancel / generation-scope pending restoreAfterActionableClose work across session boundaries before reopening the latch
• Tag arm children with sessionGeneration and ignore delayed close events from a pre-/new child

Scope

Pure upstream fix only (extension + docs note + tests). No ADO forge-adaptation commits.

Test plan

• tests/fm-pi-watch-extension.test.sh — session_start clears shutdown latch and quiet-rearms
• Restoration-cancellation and stale-child-close regressions added by review fixes
• no-mistakes local pipeline: intent/rebase/review/test/document/lint/push passed

Pipeline

Updates from git push no-mistakes

[moj02090]

hmm I used no mistakes?