Bump estimatedBytesPerCommit from 8 KiB to 64 KiB

8 KiB was inherited from the old probe-based planner where it was just an initial guess that probing would refine. Without probing, it needs to stand on its own as the batch count estimate. At 8 KiB, linux (~66 KiB/commit real) estimated 1 batch when the answer is ~3, forcing the PACK header pre-check to do all the work through repeated subdivisions.

64 KiB is a better general-purpose constant:

• Small repos (~5 KiB real): still estimates 1 batch (correct, harmless)
• Linux kernel (~66 KiB real): estimates 3 batches at 2 GiB limit (accurate)
• Blob-heavy monorepos (~120 KiB real): ~2x under, one PACK header split

The PACK header pre-check and target-rejection retry remain as safety nets for the cases where even 64 KiB underestimates, but they should now trigger rarely rather than routinely.