Fixes #1612. Also carries the behavior-neutral refactor of the editor sync and completion state that #1608/#1612 exposed. Replaces #1615.

Part 1: fix for #1612 (highlighting lost after Format)

Two layers:

1. syncBindingText called the TextView-level textView.setText(...), bypassing TextViewController.setText(_:), the documented controller-level path that rebuilds the highlighter for the replacement storage. The old highlighter kept tree-sitter state for the dead document.
2. Rebuilding alone is not enough: a fresh Highlighter only queries when triggered (an edit, a frame/bounds notification, or invalidate()). Initial load works because the first layout pass fires frameDidChangeNotification; after a mid-session storage swap no trigger is guaranteed. setUpHighlighter() now ends with highlighter.invalidate(), making "after setup, the editor is highlighted" an invariant instead of a timing accident.

Tests: test_syncRebuildsHighlighterForReplacementStorage, test_syncQueriesHighlightsForReplacementStorage (mock provider proves the rebuilt highlighter is re-set-up AND re-queried; fails with the route-through change alone).

Part 2: refactor of text binding sync (package)

The coordinator juggled four pieces of implicit state (isUpdatingFromRepresentable, isUpdateFromTextView, lastSyncedText, textBindingTask); both recent bugs were interactions between them. Now:

• RepresentableSyncPhase: explicit origin state machine (idle / editorChangePending / applyingRepresentableValue). The mutual exclusion the booleans only implied is enforced by construction.
• TextBindingSync: single owner of writeback (sync + large-doc debounce) and push-down, carrying the documented invariants (editor wins while its edit is in flight; push-down routes through TextViewController.setText).
• Coordinator and updateNSViewController shrink to thin delegation.

All 11 binding-sync behavior tests pass unchanged in meaning (retargeted in the same commit), plus 5 new state-machine transition tests.

Part 3: refactor of completion sessions (package + app)

• SQLCompletionAdapter replaces three ad-hoc fields (currentCompletionContext that was never cleared, suppressNextCompletion, a hand-rolled debounce generation counter) with one CompletionSession carrying an explicit phase (intermediate keyword seed -> final full result), the immediate-sync-then-async-refine shape NSTextSuggestionsDelegate documents.
• Debounce dedup now uses cooperative task cancellation; cancelled requests stop at the sleep instead of computing results nobody reads.
• Apply-time trigger suppression moves into SuggestionViewModel (isApplyingCompletion around the delegate apply call), fixing it for every delegate instead of each adapter keeping its own flag.
• completionWindowDidClose() was declared on the delegate protocol but never invoked; willClose() now calls it, and the adapter uses it to end the session, so context no longer leaks across popups.

New tests: trigger-during-apply ignored, willClose notifies the delegate once. All 6 app autocomplete suites pass (366 cases).

Manual checks worth doing

Type (popup filters per keystroke), accept with Tab/Return (no flicker after accept), Format (highlighting survives), Clear, JSON viewer session.