fix: compare signature-change diffs using new-file line ranges

[carlos-alm]

Summary

• checkNoSignatureChanges(db, oldRanges, noTests) compared def.line (read from db, which reflects post-change/new-file coordinates) against diff.oldRanges (pre-change/old-file coordinates). Any diff shifting line counts (virtually all deletions/insertions) could make an untouched symbol's new line number coincidentally fall inside an old hunk's range, producing a false "signature change" violation. checkMaxBlastRadius right above it already correctly used diff.changedRanges.
• Renamed the parameter to changedRanges and fixed the call site accordingly. diff.oldRanges remains computed and is still exercised elsewhere (parseDiffOutput's own tests) — not dead.

Closes #1732
Closes #1737 (confirmed true duplicate — identical file/line/root-cause/fix)

Test plan

• npm test — full suite green (3403 passed, 0 failed)
• npm run lint — clean
• New end-to-end tests built on real parseDiffOutput-parsed diff strings, reproducing the exact codegraph check --staged: signature-change detection compares new-file line numbers against old-file diff ranges #1732/codegraph check --signatures: false positive when a diff shifts line numbers above an untouched function #1737 repro (11-line deletion shifting an untouched exported function) — confirmed NOT flagged with the fix, and confirmed the counterfactual oldRanges call WOULD have flagged it
• Manual repro: built a fresh git repo mirroring the codegraph check --signatures: false positive when a diff shifts line numbers above an untouched function #1737 scenario, ran the real CLI, confirmed fixed; reverted the call site alone and confirmed the exact false positive reproduces

Stacked on #1828 (base branch fix/issue-1731-stale-edges-hash-match) — only the check.ts/test diff is this issue's change.

[greptile-apps]

Greptile Summary

This PR fixes a coordinate-space mismatch bug in checkNoSignatureChanges: the function reads symbol line numbers from db (post-change/new-file coordinates) but was comparing them against diff.oldRanges (pre-change/old-file coordinates). Any diff that shifts total line counts — virtually every deletion or insertion — could make an untouched symbol's new-file line number coincidentally land inside an old hunk's range, producing a false "signature change" violation.

• src/features/check.ts: Renames the parameter from oldRanges to changedRanges and updates the call site in runPredicates from diff.oldRanges to diff.changedRanges (new-file/added-lines coordinates, matching db).
• tests/integration/check.test.ts: Renames local variables to match the new parameter name, adds two new end-to-end regression tests that reproduce the exact codegraph check --staged: signature-change detection compares new-file line numbers against old-file diff ranges #1732/codegraph check --signatures: false positive when a diff shifts line numbers above an untouched function #1737 scenario (11-line deletion shifting an untouched exported function), confirm the false positive with oldRanges, and verify the fix with changedRanges.

Confidence Score: 5/5

Safe to merge — the change is surgical (one parameter rename + one call-site fix), matches the existing pattern already used by checkMaxBlastRadius, and is backed by regression tests that both reproduce the original bug and verify the fix.

The fix correctly aligns the coordinate space: db line numbers are post-change, and diff.changedRanges tracks added lines in post-change coordinates, making the comparison semantically valid. The two new regression tests cover both the false-positive scenario (pure deletion shifting an untouched function into an old hunk's numeric range) and the true-positive scenario (a genuinely replaced declaration still being caught). No unintended side effects — diff.oldRanges is still populated by parseDiffOutput and exercised by its own existing tests.

No files require special attention.

Important Files Changed

Filename	Overview
src/features/check.ts	Renames oldRanges parameter to changedRanges in checkNoSignatureChanges and fixes the call site to use diff.changedRanges; adds a JSDoc comment explaining the coordinate-space requirement.
tests/integration/check.test.ts	Renames oldRanges → changedRanges throughout existing tests; adds two well-structured regression tests that both prove the pre-fix bug and confirm the fix works for pure deletions and genuine signature changes.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A[runPredicates] -->|"diff.changedRanges (new-file coords)"| B[checkNoSignatureChanges]
    B --> C{For each file in changedRanges}
    C -->|"ranges.length === 0"| D[skip file — no added lines]
    C -->|ranges present| E[Query db for exported symbols\nname/kind/file/line]
    E --> F{def.line in range?}
    F -->|"yes (post-change line overlaps added-line range)"| G[Push violation]
    F -->|no| H[Symbol untouched — skip]
    G --> I[SignatureResult: passed=false]
    H --> I
    D --> I

    style D fill:#d4edda,color:#000
    style G fill:#f8d7da,color:#000

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flowchart TD
    A[runPredicates] -->|"diff.changedRanges (new-file coords)"| B[checkNoSignatureChanges]
    B --> C{For each file in changedRanges}
    C -->|"ranges.length === 0"| D[skip file — no added lines]
    C -->|ranges present| E[Query db for exported symbols\nname/kind/file/line]
    E --> F{def.line in range?}
    F -->|"yes (post-change line overlaps added-line range)"| G[Push violation]
    F -->|no| H[Symbol untouched — skip]
    G --> I[SignatureResult: passed=false]
    H --> I
    D --> I

    style D fill:#d4edda,color:#000
    style G fill:#f8d7da,color:#000

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_{Reviews (3): Last reviewed commit: "fix: remove leftover duplicate importNam..." | Re-trigger Greptile}

[github-actions]

Codegraph Impact Analysis

2 functions changed → 5 callers affected across 4 files

• checkNoSignatureChanges in src/features/check.ts:311 (4 transitive callers)
• runPredicates in src/features/check.ts:479 (4 transitive callers)