Use a per-board save queue instead of a process-wide one

[lovasoa]

Use a per-board save queue instead of a process-wide one

saveBoard ran every board's save through a single global SerialTaskQueue, so a slow rewrite on one board blocked unrelated boards from persisting and delayed shutdown saves. Each board only ever writes its own files, so the lane only needs to be per board.

The queue now lives on each BoardData instance (data.mjs), so it is GC'd with the board (no global Map to clean up). saveBoard now uses board.saveQueue. File writes stay safe because the queue still serializes each board's own writes, and no two boards share a file.

Proof

The test blocks board A mid-save and shows board B completes anyway, then shows two saves of the same board stay serialized:

const firstSave = firstBoard.save();   // blocked on a gate
const secondSave = secondBoard.save();
assert.deepEqual(await secondSave, { status: "saved" });
assert.deepEqual(calls, ["first:start", "second:start", "second:end"]);
// same-board: second save starts only after the first ends
assert.deepEqual(calls, ["a:start", "a:end", "b:start"]);

$ npm run test-node
tests 311 / pass 311 / fail 0
$ npm run lint
Checked 202 files in 49ms. No fixes applied.
$ npm run typecheck   # tsc, no output = clean