Pre-check PACK header object count before pushing oversized batches

When a batch's FetchPack returns, peek at the first 12 bytes of the pack stream to read the PACK header's object count. Multiply by 750 bytes/object (a conservative compressed-object average) to estimate total pack size. If the estimate exceeds BatchMaxPack, subdivide immediately and re-fetch smaller ranges — avoiding a multi-GiB transfer that the target would reject anyway.

This is a zero-cost optimization: 12 bytes read + one multiplication. The PACK header is prepended back to the reader via MultiReader so the push receives a valid packfile if the check passes.

Combined with the post-push subdivide from the previous commit, the batch loop now has two layers of protection:

1. Pre-push: PACK header estimate catches obviously-oversized batches before any significant transfer (this commit).
2. Post-push: target body-limit rejection triggers subdivide for batches that passed the estimate but still exceeded the target's actual limit (previous commit).

For the linux kernel against a 2 GiB target:

• Estimate plans 3 batches (even by commit count)
• Batch 3 fetches, PACK header shows 6.5M objects × 750 = ~4.9 GiB
• 4.9 GiB > 2 GiB BatchMaxPack → subdivide before pushing
• Re-fetch two halves (~3.2M objects each, ~2.4 GiB)
• If still over, subdivide again — converges in O(log n) splits