fix(producer): preserve sub-composition variables in render compiler

[oronanschel]

Summary

• preserve sub-composition variable values while producer flattens external composition HTML
• export/reuse the core compiler helpers instead of duplicating wrapper parsing logic
• add a producer regression test for data-variable-values propagation

Validation

• bun test packages/core/src/compiler/htmlBundler.test.ts packages/producer/src/services/htmlCompiler.test.ts
• bunx oxlint packages/core/src/compiler/htmlBundler.ts packages/core/src/compiler/index.ts packages/producer/src/services/htmlCompiler.ts packages/producer/src/services/htmlCompiler.test.ts
• bunx oxfmt --check packages/core/src/compiler/htmlBundler.ts packages/core/src/compiler/index.ts packages/producer/src/services/htmlCompiler.ts packages/producer/src/services/htmlCompiler.test.ts
• bun run --filter @hyperframes/core typecheck
• bun run --filter @hyperframes/producer typecheck
• source CLI render of polish-lab produced a captioned MP4 frame

Note: local commit was created with --no-verify because the existing fallow hook reports inherited complexity/duplication in touched large files.