fix(producer): preserve nested entry asset base

[miguel-heygen]

What

Direct --composition compositions/scene.html entries now keep the same sibling-asset resolution as mounted sub-compositions. A sibling image.png is emitted as compositions/image.png, while an existing project-root asset remains root-relative.

Why

The compiler relocates every entry document to compiled/index.html. Direct nested entries previously kept plain relative URLs unchanged, so the render server requested /image.png and returned 404 even when compositions/image.png existed.

How

Nested entry documents inside the project are rebased before compilation with the established mounted-subcomposition path resolver. The pass covers src/href, inline styles, and stylesheet url(...) values. Generated standalone wrappers outside the project are skipped, which prevents double rebasing of the existing mounted flow.

Test plan

• Regression test covers a direct nested entry with both a sibling image and a project-root image.
• Full producer HTML compiler suite passes (132/132).
• Shared path-rewriter suite passes (11/11).
• Producer typecheck, oxlint, and oxfmt checks pass.
• Documentation updated (not applicable).

[miga-heygen]

Reviewed the diff plus the underlying rewriteAssetPath/rewriteAssetPaths/rewriteCssAssetUrls helpers in packages/parsers/src/rewriteSubCompPaths.ts. Traced the added regression test by hand: for a direct nested entry at compositions/scene.html, a sibling compositions/sibling.png gets rewritten to compositions/sibling.png (sibling-exists probe hits), while a project-root root.png is left unchanged (sibling probe path compositions/root.png doesn't exist on disk, so it falls through to the original value) — matches the PR's stated sibling-first/project-root-fallback rule and the existing mounted-subcomposition behavior. The isPathInside(htmlPath, projectDir) guard correctly skips generated standalone wrappers living outside the project, avoiding double-rewriting of the already-handled mounted flow. Logic is consistent with the reused helpers rather than a parallel reimplementation. Approving.