fix: auto-upgrade marker no longer masks newer remote versions

[gregario]

Problem

When a just-upgraded-from marker file persists across sessions, gstack-update-check short-circuits at Step 2 (line 112–122) — it writes UP_TO_DATE to cache and exits immediately, never fetching the remote VERSION. Users silently stay on old versions, potentially missing dozens of commits.

Reported in #515: a user upgraded from 0.9.3.0 → 0.9.4.0, then missed 42 commits because the marker caused Step 4 (remote fetch) to be skipped entirely.

Root cause

# Step 2 — before fix
if [ -f "$MARKER_FILE" ]; then
  ...
  echo "UP_TO_DATE $LOCAL" > "$CACHE_FILE"  # ← premature
  ...
  exit 0  # ← never reaches remote check
fi

The script assumes "just upgraded = up to date", which isn't true if the remote advanced between the upgrade and the next session.

Fix

Remove the exit 0 and the premature UP_TO_DATE cache write. The script now:

1. Consumes and deletes the marker (same as before)
2. Clears the snooze file (same as before)
3. Outputs JUST_UPGRADED for the preamble (same as before)
4. Falls through to Step 3 (cache check) → Step 4 (remote fetch)

If no further updates exist, Step 4 writes UP_TO_DATE to cache as normal. If a newer version landed, the user sees both JUST_UPGRADED and UPGRADE_AVAILABLE in the same session.

Changes

• bin/gstack-update-check — remove early exit and premature cache write from Step 2 (+2/−3 lines)
• browse/test/gstack-update-check.test.ts — add two regression tests:
  • Path C2: marker present + remote has newer version → outputs both notices, cache reflects UPGRADE_AVAILABLE
  • Path C3: marker present + remote matches local → outputs JUST_UPGRADED, cache reflects UP_TO_DATE

Test plan

• All 35 existing + 2 new tests pass (bun test browse/test/gstack-update-check.test.ts)
• Full bun test passes (only pre-existing bug: package.json version out of sync with VERSION file on main #399 version mismatch fails — unrelated)
• Existing "just-upgraded clears snooze file" test still passes
• Existing "outputs JUST_UPGRADED and deletes marker" test still passes (falls through to remote fetch, which writes UP_TO_DATE on unreachable remote)

Fixes #515

[gregario]

Merged via #552. Closing.