Fix Google Calendar sync race condition and recurring completion date bug

[martin-forge]

Summary

Two independent Google Calendar sync reliability fixes:

• Debounce stale metadata cache: TaskCalendarSyncService.updateTaskInCalendar discarded the explicit task payload during the 500ms debounce and re-fetched from cacheManager.getTaskInfo, which often returned stale data because Obsidian's metadata cache hadn't finished indexing. Fixed by caching the authoritative task payload in a pendingTasks Map during the debounce window.

• Recurring completion records wrong occurrence: toggleRecurringTaskComplete and toggleRecurringTaskSkipped defaulted to getTodayLocal() when no explicit date was passed, even for scheduled-anchor recurring tasks. Completing a task late (e.g. Saturday's task on Sunday) recorded Sunday in complete_instances — but complete_instances is an occurrence index, not a completion timestamp, so this left Saturday's occurrence open and sent the wrong EXDATE to Google Calendar. Fixed by defaulting to task.scheduled for scheduled-anchor tasks, so the correct occurrence is marked as done.

Issue linkage

• Closes [Bug]: Race condition in TaskCalendarSyncService debounce fetching stale metadata cache #1763
• Closes [Bug]: Recurring tasks completed after scheduled date do not process into next available recurrence #396
• Closes [Bug]: Incorrect complete_instance date on recurring tasks #574
• Related to [Bug]: Completing a repeating task that's overdue doesn't work from the note #1594 — this PR fixes the overdue recurring wrong-date behaviour, not the separate note-view/calendar occurrence-context bug described there.

Test plan

• npx jest tests/services/TaskCalendarSyncService.test.ts — debounce uses explicit payload, not stale cache
• npx jest tests/unit/issues/issue-396-recurring-late-completion-wrong-date.test.ts — scheduled-anchor defaults to scheduled date; completion-anchor still defaults to today; undefined anchor defaults to scheduled
• npx tsc --noEmit — clean typecheck
• npm run build — clean build
• Validated locally: rapid MCP task updates reflect correct dates in Google Calendar; late-completed recurring tasks mark the scheduled occurrence as done rather than recording a non-existent occurrence on the completion date

CI note

The test (20) check fails on Could not locate module tasknotes-nlp-core — this is a pre-existing repo-wide CI issue that is also affecting other PRs (for example #1677), and it was not introduced by these changes.

[martin-forge]

The only failing check here is the known repo-wide tasknotes-nlp-core CI wiring issue in test (20), not a failure introduced by this branch.

I re-ran the targeted tests for the changes in this PR, plus npx tsc --noEmit and npm run build, locally and all of those passed.

If branch protection allows, I believe this PR is ready to merge as-is. If helpful, I’m also happy to put together a separate PR for the CI issue rather than fold that into this change.