fix(queue): do not dissolve a batch whose checks do not exist yet

[matt82198]

Fourth and final cause of the merge-queue stall. With #740 merged the advancer now builds and pushes an integration branch correctly — observed live: batch #741 (integrate/q-1785745378, members #719 + #725) was created successfully. It was then dissolved on the very same pass:

batch #741 (integrate/q-1785745378) dissolved (required check(s) absent from rollup: windows)

The batch PR was seconds old. GitHub creates check runs asynchronously after gh pr create, and the windows aggregator only appears once its shards exist — so the rollup was incomplete through no fault of the batch. Dissolution evicts every member (queue-rejected), the next pass rebuilds the same batch, and it dissolves again. That is the loop behind #727 / #729 / #730 / #731 / #732 / #733 / #736 / #737 — eight batches, nothing merged, every pass reporting PASSED. The exception ledger shows the same signature on the originals: all members individually green but #736 was red: required check(s) absent from rollup: windows.

The confusion

"Absent" was treated as identical to "red". Those need to differ by direction:

• Merging — absent must never read GREEN. Unchanged. required_checks_green still answers not_green for an incomplete rollup, so nothing new can merge. A test asserts a young incomplete batch is still never merged.
• Dissolving — needs positive evidence of failure: a required context that is present and concluded not-green.

batch_checks_not_yet_created() answers True only when all of:

1. no required context has actually concluded not-green,
2. at least one required context is absent, and
3. the batch PR is younger than BATCH_CHECK_GRACE_S (30 min).

A concluded FAILURE dissolves immediately regardless of age; an absence past the window dissolves exactly as before; an unreadable createdAt is not treated as young. The window is a delay, never an amnesty.

Tests

128 in tests/test_merge_queue.py (6 new): young-waits, old-still-dissolves, young-with-real-failure-still-dissolves, unknown-timestamp-dissolves, grace-never-merges, and createdAt is actually requested in PR_FIELDS (the check is inert otherwise). The two behavioural ones fail against the pre-fix module.

Stall series

• fix(queue): unstall batch construction -- porcelain parse + pre-push regeneration #740 (merged) — porcelain parse off-by-one + pre-push regeneration
• this PR — premature dissolution
• guard: wire 4 orphaned gate tools into CI + document 9th hook check #712 held queue-rejected until guard: import_resolution_check learns the sanctioned sys.path idiom (60% FP -> 0, zero coverage loss) #724 lands (wires a guard whose checker flags merge_queue.py:68-69)

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