docs(php): document '...' vs 'params' variadic-marker asymmetry (U5)

The shared narrowOverloadCandidates pass in scope-resolution/passes/
overload-narrowing.ts checks for 'params'/'params ' as the variadic
marker - the C# 'params' keyword convention. PHP uses '...' instead,
matching its source-language syntax. The discrepancy is harmless in
practice because PHP variadic methods set parameterCount=undefined,
which skips the 'max !== undefined && argCount > max' gate that hosts
the 'params' check entirely - so the C# branch is dead code for PHP.

Document the asymmetry at both sites with cross-references so a future
contributor reading either file lands on the full picture:

- overload-narrowing.ts:48 area: explains the 'params' branch is C#-
  specific and warns that adding new markers requires auditing each
  adapter's arity-metadata.ts.
- languages/php/arity-metadata.ts:46 area: explains PHP uses '...'
  intentionally and that the shared pass's 'params' branch is dead
  code for PHP because of the parameterCount=undefined setting.

No behavior change. Existing PHP variadic-resolution tests already
exercise the live '...' path.

gitnexus/src/core/ingestion/languages/php/arity-metadata.ts

@@ -43,6 +43,17 @@ export function computePhpArityMetadata(fnNode: SyntaxNode): PhpArityMetadata {
     }
     if (p.type !== null) types.push(p.type);
   }
+  // PHP variadic marker convention: append the literal '...' string to
+  // `parameterTypes`. This is intentionally DIFFERENT from C#, which uses
+  // the literal 'params' (its source-language keyword). The shared
+  // `narrowOverloadCandidates` pass in `scope-resolution/passes/overload-
+  // narrowing.ts` checks for the C# 'params' marker — that branch is
+  // dead code for PHP because PHP variadic methods set `parameterCount
+  // = undefined` (see line below), which skips the `max !== undefined`
+  // gate that hosts the 'params' check. PHP's actual variadic-aware
+  // arity logic lives in `phpArityCompatibility` (arity.ts) and now
+  // also in `phpEmitUnresolvedReceiverEdges` (scope-resolver.ts), both
+  // of which check `'...'`. Finding 9 of PR #1497 adversarial review.
   if (hasVariadic) types.push('...');
 
   const total = params.length;

gitnexus/src/core/ingestion/scope-resolution/passes/overload-narrowing.ts

@@ -43,6 +43,16 @@ export function narrowOverloadCandidates(
           const max = d.parameterCount;
           const min = d.requiredParameterCount;
           if (max !== undefined && argCount > max) {
+            // Variadic marker check is C#-specific (the 'params' keyword).
+            // Other languages use their own marker — PHP uses '...' (see
+            // `languages/php/arity-metadata.ts:46`), Python uses '*args'-
+            // shaped metadata that lives outside `parameterTypes` entirely.
+            // This branch is dead code for those languages because they
+            // set `parameterCount = undefined` for variadic functions,
+            // which keeps `max` undefined and skips this check entirely.
+            // Adding new variadic markers here changes behavior for those
+            // other languages too — don't extend without auditing each
+            // adapter's `arity-metadata.ts`. Finding 9 of PR #1497.
             const variadic =
               d.parameterTypes !== undefined &&
               d.parameterTypes.some((t) => t === 'params' || t.startsWith('params '));