check HEAD --signatures flags newly ADDED exported functions (should only flag modifications) #2674

Summary

codegraph check HEAD reports [FAIL] signatures when an exported function is newly added (not modified). The predicate help text says "Assert no exported function/method/class declaration lines were modified" — additions are not modifications, yet they are flagged, and the run exits 1. This blocks additive-extension work.

Minimal repro

1. In a tracked JS file, append a new exported function (no existing lines touched):

   export function brandNewThing(x) { return x; }

2. codegraph build . --no-incremental
3. codegraph check HEAD

   [PASS] cycles
   [FAIL] signatures
          brandNewThing (function) at <file>:NNN
   [PASS] boundaries
   3 predicates | 2 passed | 1 failed    (exit 1)
   

Observations (all reproduced)

• Inserting the function before an existing export and appending at EOF both fail → not a line-offset artifact.
• Declaring it as export const brandNewThing = (x) => x; is also classified function and flagged → no syntax workaround.
• codegraph check HEAD --json reports the symbol with "passed": false; the symbol is absent at HEAD (e.g. git show HEAD:<file> | grep -c brandNewThing → 0) and has 0 transitive callers (codegraph fn-impact brandNewThing → 0).
• Manifesto mode (codegraph check, no ref) exits 0.

Version

@optave/codegraph 3.17.0 (native engine unavailable → wasm). Report pointer: dist/features/check.js:439 (checkNoSignatureChanges) flags any changed range covering an exported def line without distinguishing add from modify.

Requested

Discriminate additions from modifications: only flag a symbol whose declaration exists at the base ref and changed at the same line/signature; treat symbols that are new at HEAD as non-violations. (Or add an explicit opt-out flag for additive changes.)