refactor(core): apply /simplify findings on validation PR

- core.types.ts: export COMPOSITION_VARIABLE_TYPES, a runtime tuple of
  every CompositionVariableType variant guarded by `as const satisfies
  readonly CompositionVariableType[]`. Adding a new variant to the union
  without also adding it to the tuple becomes a compile error rather
  than silent drift in callers that maintain their own list.
- composition.ts (lint rule): the local
  `new Set(["string","number","color","boolean","enum"])` now derives
  from COMPOSITION_VARIABLE_TYPES instead of duplicating the list.
- index.ts: export COMPOSITION_VARIABLE_TYPES alongside the rest of the
  variable type guards.

Reuse + efficiency reviews otherwise clean. The other reuse finding
(loadProjectHtml helper to dedupe readFileSync + ensureDOMParser across
3 callers) is real but reaches files outside this PR's scope; it's a
better fit as a follow-up cleanup once the variable-feature stack lands.

All 48 composition lint tests + 49 core suite tests still green.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/core/src/lint/rules/composition.ts

@@ -1,5 +1,6 @@
 import type { LintContext, HyperframeLintFinding } from "../context";
 import { findHtmlTag, readAttr, readJsonAttr, truncateSnippet } from "../utils";
+import { COMPOSITION_VARIABLE_TYPES } from "../../core.types";
 
 // Agent guidance thresholds: warning-only nudges for files/tracks that become hard
 // to inspect and revise reliably in a single composition.
@@ -475,7 +476,7 @@ export const compositionRules: Array<(ctx: LintContext) => HyperframeLintFinding
     }
 
     const findings: HyperframeLintFinding[] = [];
-    const knownTypes = new Set(["string", "number", "color", "boolean", "enum"]);
+    const knownTypes = new Set<string>(COMPOSITION_VARIABLE_TYPES);
     for (let i = 0; i < parsed.length; i += 1) {
       const entry = parsed[i];
       if (!entry || typeof entry !== "object" || Array.isArray(entry)) {