fix(station-boards): push last snapshot on subscribe when one exists

[ciotlosm]

Symptom

Switching between views (e.g. station view -> home, or any pair of views that use createStationBoardsController) showed scheduled-only vehicles for up to one poll cycle (15 s today) before live GPS buses reappeared, even when data was already flowing on the previous view.

Root cause

subscribeStationBoards in src/lib/workers/gtfs/stationSubscribers.ts explicitly skipped the late-subscribe catch-up. PR #454 removed it on 2026-07-22 to fix an incoming-first flicker in StationCard groups on cold start, but the regression is that the new view's subscriber now waits for the next tickLive to receive its first push.

PR #454 was correct for the first-ever subscription (before the first tickLive has run, getReconciledSnapshot() returns null, and pushing the empty scheduled-only board does cause flicker). It was wrong for every subsequent subscription, where a non-null snapshot already exists and is what the stream is currently broadcasting.

Fix

In subscribeStationBoards, push the last good snapshot on subscribe but only when getReconciledSnapshot() is non-null. The cold-start path is unchanged (the null check skips the push), so the PR #454 flicker fix is preserved. Every subsequent subscription paints within a microtask with the same merged-with-GPS data the stream is currently showing.

const snap = getReconciledSnapshot();
if (snap) void pushOne(sub, snap);

The new view now reads the most recent merged snapshot the moment its controller mounts. The next tickLive (within livePollMs = 15 s) replaces it with a fresher one.

Verification

• 386 tests pass (npx vitest run)
• svelte-check 0 errors
• Diff is 16 insertions / 6 deletions in one file (focused; no whole-file reformat)

Related

PR #454 (fix: eliminate incoming-first flicker in StationCard groups) is the fix this restores the missing half of. The two together are the complete solution: no flicker on cold start, no 15 s wait on view switch.