chore(theme): drop NULL_COLOR symbol indirection in vscode-color-registry

[dormouse-bot]

Surfaced by the nightly code-quality survey.

vscode-color-registry.ts had a three-piece symbol layer — NULL_COLOR (a Symbol), the ResolvedColor type (string | typeof NULL_COLOR), and registryDefault() returning ResolvedColor — whose only purpose was to let registryDefaultValue() immediately collapse the symbol back to string | null:

export function registryDefault(name, themeKind): ResolvedColor {
  const defaults = REGISTRY_DEFAULTS[name];
  if (!defaults) return NULL_COLOR;
  const value = defaults[themeKind];
  if (value) return value;
  return NULL_COLOR;
}
export function registryDefaultValue(name, themeKind): string | null {
  const value = registryDefault(name, themeKind);
  return value === NULL_COLOR ? null : value;
}

The symbol never escaped the module — nothing outside imports NULL_COLOR, ResolvedColor, or registryDefault (the resolver imports only registryDefaultValue). The indirection added a layer with no caller benefit, so this folds the lookup directly into registryDefaultValue and drops the dead symbol/type/function:

export function registryDefaultValue(name, themeKind): string | null {
  const defaults = REGISTRY_DEFAULTS[name];
  if (!defaults) return null;
  return defaults[themeKind] || null;
}

Behavior is unchanged (defaults[themeKind] || null maps empty-string and null to null, exactly as the old if (value) … else NULL_COLOR did). No new test is added because this is a pure refactor with no behavior change; the existing theme suite (vscode-color-resolver.test.ts and the rest of src/lib/themes/, 48 tests) exercises registryDefaultValue and passes unchanged. tsc -b is clean.

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