[Routine - VT] fix(provider): flush pending debounced save on deactivation

[winterdrive]

Pre-flight Check

Open PRs / active routine branches checked before starting (Phase 1):

PR	Branch	Files touched
#82	routine/vt-fix-validatejson-undefined-files-260709	mcp-server/src/server.ts (validate_json_structure, files-iteration branch)
#81	routine/vt-fix-autogroup-bookmarks-260708	src/core/AutoGrouper.ts (bookmark-move on auto-group)
#80	routine/vt-fix-jumptobookmark-clamp-260707	src/commands.ts (jumpToBookmark position clamp, ~L1188-1200)
#79	routine/vt-fix-filemanager-relpath-260706	src/core/FileManager.ts (addFilesToGroup/removeFilesFromGroup)
#78	routine/vt-fix-bookmark-stale-groupidx-260703	src/provider.ts (getChildren bookmark-render branch, ~L1852-1860)
#77	routine/vt-fix-treeview-listener-disposal-260702	src/extension.ts (event-listener subscriptions + revealTimeout disposal, ~L130-260)
#76	release/0.7.6-260701	release PR, not a code-overlap concern

This PR does not overlap any of the above:

• src/provider.ts: adds a new method flushPendingSave() right after the existing saveGroups() debounce helper (~L332), nowhere near the bookmark-render branch [Routine - VT] fix(provider): guard against stale groupIdx when rendering file bookmarks #78 touches (~L1852).
• src/extension.ts: adds one context.subscriptions.push(...) at line ~120, immediately after treeView.description = ..., which is before the listener-registration block [Routine - VT] fix(extension): dispose treeView listeners and pending reveal timer #77 modifies (starts ~L137). No line overlap with [Routine - VT] fix(extension): dispose treeView listeners and pending reveal timer #77's diff hunks.
• No other open PR touches these two files at all.

Changes

TempFoldersProvider debounces disk writes: every group/file/bookmark edit schedules a saveGroupsImmediate() call 500ms later via setTimeout, coalescing rapid edits into one write. However, nothing ever flushed that pending timer on extension deactivation. If a user made an edit and closed VS Code within that 500ms window, the timer was simply discarded by the JS runtime and the change was silently lost — never written to .vscode/virtualTab.json.

Fix:

• src/provider.ts: new public flushPendingSave() method — clears the pending debounce timer (if any) and calls the existing synchronous saveGroupsImmediate() directly.
• src/extension.ts: registers context.subscriptions.push({ dispose: () => provider.flushPendingSave() }) right after the tree view is created, so VS Code invokes it automatically on deactivation.

This does not modify any command registrations, contributed configuration keys, dependencies, or the tab-group serialization/storage format — it only ensures an already-scheduled write actually happens before shutdown.

Safety Verification

Commands run locally, from the repo root:

• npx tsc -p ./ → passed, no errors.
• npm run test (runs tsc -p ./ && jest --runInBand) → 25 test suites, 175 tests, all passed.

No new test was added: the existing unit-test suite has no vscode-module mock/harness for instantiating TempFoldersProvider (all current tests exercise pure src/core/* logic), so a proper test for this deactivation-hook fix would require scaffolding beyond the scope of a "smallest practical fix" routine PR. This mirrors the precedent set by #77/#78, which also shipped one-line defensive fixes to extension.ts/provider.ts without new tests.

CI / Release Gate Note

This is a daily routine Draft PR. Package version bump and CHANGELOG.md consolidation are intentionally deferred to the weekend release/integration PR. If CI fails solely due to the repository's version-bump release gate, that is expected release-readiness behavior for a routine PR, not a code validation failure.

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