Decline the pre-flight subdivide instead of skipping it, and pin the behaviour

Skipping checkPackSizeAndSubdivide for an unsplittable span also skipped the only thing that parses the pack header, so packObjectCount fell to 0 on exactly the push this feature exists for: object_count and estimated_bytes logged as zero on both lines, and calibrateBytesPerObject got a zero denominator. The calibration loss was inert — an indivisible span never takes the growth path now, so no later iteration consumes it — but the logs are the ones an operator reads when a giant commit stalls.

The call is made unconditionally again; the callback declines instead. Same outcome for subdivision, header still parsed, counts still real.

The skip was also unpinned: removing it passed the entire suite, which stands out in a change whose every other claim is mutation-verified. Now pinned by fetch count — 5 commits into 4 batches gives gaps of 1,1,1,2, and a header declaring 200 objects makes the estimate exceed the budget so the pre-flight actually fires. Nine fetches with the decline, eleven without: each wasted fetch is an indivisible span re-planned before pushing.

Docs: the sentinel's comment had a 107-character line in a block that wraps at 79; observedSubdivisionFactor's doc block sat orphaned above a different function and had started documenting nextBudgetProvenance, so it moves down to the function it describes; and the client comment claimed Counts describe attempted work when the error path does not populate them at all — Applied is 0 even where refs were pushed before the failure. That comment is the mirror worker's contract, so it now says to read Refs and ignore Counts.