Surface bootstrap pack subdivisions as inline notices

When a bootstrap push hits the target body limit, the planner silently splits the remaining work into more packs — first 1 → 2, then on next rejection 2 → 4, and so on. Until now the only trace of this was a slog line behind --verbose, so users running with --progress just saw the pack denominator jump from 2 to 4 with no explanation.

Add a one-line notice at each subdivision point so the reason is visible inline with progress. Three trigger sites: the one-shot → batched fallback, the pre-push header-estimate split, and the post-rejection split. Messages flow through a new OnNotice hook on bstrap.Params, then through syncSession.notice, which routes them through progressReporter.notify (clears the live frame, prints the message, redraws on next tick) when --progress is on and falls back to plain stderr otherwise.