export: fix render-ticket classification; export any sequence

Two export killers fixed and the feature surfaced properly:

- TicketArena::allocate() reaps finished fire-and-forget slots on every
  new submit, but RenderTask classified finished tickets by asking the
  arena for their kind/time afterwards - any ticket that completed
  before the next submit (all of them on the inline backend) failed
  with 'Render ticket reported an unexpected timestamp' and the export
  died after writing a header-only file. RenderTask now records each
  ticket's delivery key at submit time instead.
- File > Export becomes 导出序列 (all 8 locales): the dialog offers a
  sequence picker (any sequence in the project, the current one
  preselected) instead of silently exporting the current one, and the
  project bin's sequence context menu gains 导出序列 opening the same
  dialog. Engines expose sequence_entries / current_sequence_id /
  start_export_of.
- The export-project dialog drops its dead hand-typed path field (OK
  already asked the platform save dialog).
- New end-to-end test: a one-second sequence exports to a real
  H.264/AAC MP4 that probes and decodes back to the clip's content.

crates/oak-task/src/render.rs

@@ -628,6 +628,7 @@ impl RenderTask {
 		time: Rational,
 		dispatch: *mut RenderDispatch,
 		in_flight: &mut Vec<TicketId>,
+		ticket_keys: &mut HashMap<TicketId, (i32, i64, i64)>,
 	) -> Result<()> {
 		unsafe {
 			(&*dispatch).running.fetch_add(1, Ordering::SeqCst);
@@ -635,6 +636,10 @@ impl RenderTask {
 		match self.submit_video_ticket(arena, time, dispatch) {
 			Ok(id) => {
 				in_flight.push(id);
+				ticket_keys.insert(
+					id,
+					(TICKET_VIDEO, time.numerator(), time.denominator()),
+				);
 				Ok(())
 			}
 			Err(e) => {
@@ -654,6 +659,7 @@ impl RenderTask {
 		range: TimeRange,
 		dispatch: *mut RenderDispatch,
 		in_flight: &mut Vec<TicketId>,
+		ticket_keys: &mut HashMap<TicketId, (i32, i64, i64)>,
 	) -> Result<()> {
 		unsafe {
 			(&*dispatch).running.fetch_add(1, Ordering::SeqCst);
@@ -661,6 +667,14 @@ impl RenderTask {
 		match self.submit_audio_ticket(arena, range, dispatch) {
 			Ok(id) => {
 				in_flight.push(id);
+				ticket_keys.insert(
+					id,
+					(
+						TICKET_AUDIO,
+						range.in_().numerator(),
+						range.in_().denominator(),
+					),
+				);
 				Ok(())
 			}
 			Err(e) => {
@@ -673,31 +687,20 @@ impl RenderTask {
 	}
 
 	/// Map a finished ticket back to its delivery slot: the audio slot for
-	/// audio tickets, the matching frame slot for video tickets.
+	/// audio tickets, the matching frame slot for video tickets. The key
+	/// comes from `ticket_keys` (the submitter-side record) — NOT from the
+	/// arena: the arena reaps finished fire-and-forget slots on the next
+	/// `allocate()`, so a ticket that completed before the next submit is
+	/// already gone from the arena's own map (the "Render ticket reported
+	/// an unexpected timestamp" failure that killed every export).
 	fn classify_ticket(
 		&self,
-		arena: &TicketArena,
+		ticket_keys: &HashMap<TicketId, (i32, i64, i64)>,
 		id: TicketId,
 		slot_by_key: &HashMap<(i32, i64, i64), usize>,
 	) -> Option<usize> {
-		let kind = arena.kind(id)?;
-		if kind == TICKET_AUDIO {
-			let range = arena.range(id)?;
-			slot_by_key
-				.get(&(
-					TICKET_AUDIO,
-					range.in_().numerator(),
-					range.in_().denominator(),
-				))
-				.copied()
-		} else if kind == TICKET_VIDEO {
-			let time = arena.time(id)?;
-			slot_by_key
-				.get(&(TICKET_VIDEO, time.numerator(), time.denominator()))
-				.copied()
-		} else {
-			None
-		}
+		let key = ticket_keys.get(&id)?;
+		slot_by_key.get(key).copied()
 	}
 
 	/// Drive the whole render: keep up to `max_inflight` frame tickets in
@@ -815,14 +818,21 @@ impl RenderTask {
 		let mut next_slot = 0usize;
 		// Reorder buffer: finished tickets not yet deliverable.
 		let mut pending: HashMap<usize, (TicketId, TicketResult)> = HashMap::new();
+		// Submitter-side ticket id -> delivery-slot key (the arena reaps
+		// finished fire-and-forget slots; this record is authoritative).
+		let mut ticket_keys: HashMap<TicketId, (i32, i64, i64)> = HashMap::new();
 		let mut progress_counter = 0.0;
 		let mut result: Result<()> = Ok(());
 
 		// Queue audio first (mirrors the C++ order).
 		if self.audio_enabled && result.is_ok() {
-			if let Err(e) =
-				self.start_audio_ticket(&arena, self.export_range, dispatch, &mut in_flight)
-			{
+			if let Err(e) = self.start_audio_ticket(
+				&arena,
+				self.export_range,
+				dispatch,
+				&mut in_flight,
+				&mut ticket_keys,
+			) {
 				result = Err(e);
 			}
 		}
@@ -835,6 +845,7 @@ impl RenderTask {
 					frame_times[next_frame_index],
 					dispatch,
 					&mut in_flight,
+					&mut ticket_keys,
 				) {
 					result = Err(e);
 					break;
@@ -847,7 +858,7 @@ impl RenderTask {
 			// Drain the completion queue into the reorder buffer.
 			while let Some((id, ticket_result)) = dispatch_ref.pop_finished() {
 				consumed += 1;
-				match self.classify_ticket(&arena, id, &slot_by_key) {
+				match self.classify_ticket(&ticket_keys, id, &slot_by_key) {
 					Some(slot_index) => {
 						pending.insert(slot_index, (id, ticket_result));
 					}
@@ -965,6 +976,7 @@ impl RenderTask {
 					frame_times[next_frame_index],
 					dispatch,
 					&mut in_flight,
+					&mut ticket_keys,
 				) {
 					result = Err(e);
 					break;