refactor(scope-resolution): align Const label emission with legacy DAG (PR #1718 review F1)

Eliminates the architectural fragility surfaced by PR #1718's adversarial review
Finding 1. Previously, normalizeNodeLabel('const') returned 'Variable' while
the legacy DAG parse phase emits 'Const' graph nodes (via @definition.const
capture for lexical_declaration). PR #1718's Case 5 value-receiver bridge
resolved correctly only because resolveDefGraphId happened to fall back to
simpleKey after the qualified-key miss — accidental correctness.

After this change, scope-resolution defs for `const x = ...` declarations
report def.type === 'Const', matching the graph node label. resolveDefGraphId's
qualified-key path now hits on the first try; the simple-key fallback is no
longer load-bearing for value receivers and can be tightened in future without
silently breaking Case 5.

Audit completeness verification:
- Grep `\bVariable\b` across src/core/ingestion/scope-resolution/ surfaced two
  consumer sites that already accept both labels: reconcile-ownership.ts:101+168
  (`def.type === 'Variable' || def.type === 'Const' || ...`) and
  walkers.ts:207 isOwnableValueLabel (`Const | Variable | Property | Static`).
  No language hook in src/core/ingestion/languages/ branches on
  `def.type === 'Variable'` for what's actually a const declaration.
- Sentinel stress test (the full unit + integration suite run with the
  renamed label in place): 6137/6137 unit tests pass; 2967/2967 integration
  tests pass. One pre-existing Windows-only flake on worker-pool.test.ts when
  run alongside the full integration suite (passes 28/28 in isolation,
  unrelated to scope-extractor — same flake observed before this diff).

The variable mapping (`'variable' → 'Variable'`) is preserved for `var`
declarations, matching the legacy DAG's `@definition.variable` capture for
variable_declaration. The split now mirrors the parse-phase capture
distinction exactly.

Per plan docs/plans/2026-05-21-002-feat-pr1718-followups-class-instance-and-label-normalization-plan.md
U4 + U5. T1 (class-instance singleton resolution from issue #1358's second
sub-case) is deferred to a standalone pre-plan investigation, not shipped
here.

gitnexus/src/core/ingestion/scope-extractor.ts

@@ -693,8 +693,14 @@ function normalizeNodeLabel(kindStr: string): SymbolDefinition['type'] | undefin
     case 'property':
       return 'Property';
     case 'variable':
-    case 'const':
       return 'Variable';
+    // `const` / `let` declarations align with the legacy DAG parse phase,
+    // which emits `Const` graph nodes via `@definition.const` capture for
+    // `lexical_declaration`. Returning `'Const'` here lets resolveDefGraphId's
+    // qualified-key path succeed for value receivers without relying on the
+    // simple-key fallback (PR #1718 review Finding 1 / 2026-05-21-002 U4).
+    case 'const':
+      return 'Const';
     case 'typealias':
     case 'type_alias':
       return 'TypeAlias';