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/index.ts

@@ -38,6 +38,7 @@ export {
   CANVAS_DIMENSIONS,
   TIMELINE_COLORS,
   DEFAULT_DURATIONS,
+  COMPOSITION_VARIABLE_TYPES,
   isTextElement,
   isMediaElement,
   isCompositionElement,