Preserve attribution across commit-time line reordering

[svarlamov]

Fixes #1845

Summary

• treat checkpoint and committed snapshots containing the same line multiset as a commit-time reorder instead of three-way merging both insertion positions
• rebase checkpoint attribution through single-line moves so import sorters preserve ownership
• retain genuine observed-only reorderings when the committed side is unchanged

Root cause

When a formatter or import sorter reordered AI-added lines after the AI checkpoint but before commit, carryover reconciliation treated the old and new insertion positions as independent changes. The synthetic carryover snapshot duplicated the moved lines. Attribution followed those duplicate ghost lines into INITIAL, leaving the actual committed lines untracked even though the referenced blob still existed.

This is platform-independent and reproduces the macOS reports without requiring pre-v1.6 data or CRLF conversion.

Test coverage

• added a TestRepo integration regression covering scrambled AI imports sorted before commit
• verifies pre-commit status reports 4 AI / 0 unknown
• verifies committed blame and stats remain 4 AI / 0 unknown
• verifies a clean commit leaves no phantom INITIAL attribution
• added unit coverage for duplicate lines and observed-only reorder carryover

Validation

• task test
• task lint
• task fmt
• task build

[svarlamov]

• Preserve attribution across commit-time line reordering #1967 👈 (View in Graphite)
• main

This stack of pull requests is managed by Graphite. Learn more about stacking.

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.