Rename --batch-max-pack-bytes to --target-max-pack-bytes

With estimate-based planning, PACK header precheck, and auto-subdivide, there's no longer a reason for the user to think in terms of "batch sizes." The single meaningful knob is the target's receive-pack body limit. The system estimates checkpoint placement from it, prechecks PACK headers against it, and auto-subdivides when batches exceed it.

Rename across CLI flags, internal types, unstable API, docs, and tests: --batch-max-pack-bytes → --target-max-pack-bytes BatchMaxPackBytes → TargetMaxPackBytes BatchMaxPack → TargetMaxPack defaultAutoBatchMaxPackBytes → defaultTargetMaxPackBytes autoBatchMaxPackBytes → autoTargetMaxPackBytes

Pure rename, no behavioral changes. All tests pass.