Cover batched + AllRefs + BestEffort, polish bootstrap polish-items

The most complex --all-refs path had no end-to-end coverage: large source pack forces TargetMaxPackBytes batching, the tail phase pushes other-kind refs after checkpointed branch batches, and the target ng's the notes ref. The OnRejection callback has to flow through *Pusher into bootstrap.Params.TargetPusher's interface boundary and downgrade the rejected ref to a warning. New TestBootstrap_IntegrationAllRefsBatched- BestEffortDowngradesNg pins this — the receivePackHook only fires on the tail-phase push (so branch batches go through the real handler) and the test asserts both Warned=1 and the kept-on-target branch tip.

Plus two polish items:

• bootstrap.Result.Pushed doc: explicit that BestEffort downgrades happen at a higher layer, so direct strategy callers (currently none — only syncer wraps it) know they're seeing attempted-push count, not net.
• tailPhaseLabel: "pushing tags and other refs" → "pushing tail refs" for the mixed case. The old wording read ambiguously.