fix(review): suppress phantom [index-breaking-change] when the diff redefines the symbol

[ajianaz]

Fixes #533

What

• scan_breaking_changes now builds the post-image definition set of the diff (symbol names matched on added lines, same regex set as the removal scan, diff-global across chunks) before scanning for removals
• A removal candidate whose name still exists anywhere in the post-image is treated as signature drift / move / definition-line rewording, not a removal, and is skipped
• True removals (name absent from the post-image) are still reported — including genuine renames, where only the old name fires
• Internal scan_breaking_changes_with(conn, ...) split so the full contract is testable against an in-memory index
• 5 new tests, including the exact acceptance fixture from the issue

Why

While landing #531 the pre-commit hook reported [MAJOR] [index-breaking-change] Removing build_review_prompt breaks 2 caller(s) even though nothing was removed — a signature-only edit shows the old definition as a - line, and a stale index made it look like a phantom removal. The scanner compared removed lines against a possibly-outdated index without ever checking whether the symbol still exists in the change itself (#533).

Testing

• Acceptance fixture from the issue: signature-drift diff against a stale index with 1 caller → no finding (was: MAJOR false positive)
• Genuine removal against the same index → still fires with index-breaking-change / Major
• Rename (old_name → new_name) → fires only for old_name
• Cross-file move of a called function → no finding (post-image is diff-global)
• collect_added_definitions unit test: added names collected, removed/context names not
• cargo fmt --all -- --check clean; cargo clippy --all-targets --features tree-sitter -- -D warnings 0 warnings; cargo test --features tree-sitter 931 passed, 0 failed (909 unit + 16 CLI + 6 config)

Note: the pre-existing make_chunk test helper stores diff content with the +/- marker, while the real parser strips it — the old removal-scan tests passed vacuously against anchored regexes. New tests use a chunk_lines helper matching parser semantics.

All commits signed-off (DCO).