Fail closed on deletion-only line attribution

[hzw0813]

Summary

• reconcile zero-context current-side hunk spans with Git numstat additions before attributing current changed symbols or call sites
• fail closed when a deletion-only hunk's zero-length new-side range is represented only by Git's neighboring-line anchor
• add an end-to-end regression proving an unchanged symbol/call on that anchor is no longer reported as changed
• emit an explicit limitation when current-line attribution is withheld
• regenerate tracked dist and document the fix

Why

Git encodes a deletion-only zero-context hunk such as @@ -2 +1,0 @@ with a zero-length new-side range anchored at current line 1. ProofDiff's DiffHunk representation currently stores that as newRange: { start: 1, end: 1 }. Direct interval matching therefore treated the unchanged current line 1 as changed, which could falsely report both a parser-observed call and a current symbol as part of the change.

The fix does not guess which anchor is real code. It validates that the sum of reconstructed current-side hunk spans exactly matches Git's numstat additions; if not, current changed-symbol/call attribution is withheld and the limitation is reported. This preserves existing coverage's fail-closed philosophy without changing the public hunk schema.

Verification

• independently reproduced Git's @@ -2 +1,0 @@ deletion-only shape with an unchanged call on the anchor line
• regression asserts the exact hunk shape reaches analysis, while changedSymbols and changedCalls remain empty
• full npm test passed
• clean npm run clean && npm run build passed
• git diff --check passed
• generated dist refreshed
• temporary updater workflow removed from the final diff