Bound range-diff memory during branch moves

[svarlamov]

Fixes #2093

Summary

• cap consecutive dropped-commit mappings after exact range-diff matches, matching the existing pre-match bound
• preflight the old range and bound the new range to a verified tip suffix before spawning git range-diff
• preserve exact mappings for small moved stacks even when their shared upstream history exceeds the cap
• fail closed when either range cannot be bounded

Test coverage

• parser regression for more than 64 dropped commits after a match
• TestRepo regression proving branch moves no longer fabricate one mapping per dropped commit
• TestRepo regression proving oversized old ranges are rejected before spawning git range-diff
• TestRepo regression proving long upstream history retains moved-stack attribution
• full task test suite, task lint, and task fmt