Fix code-split alias modules and dynamic CSS imports

[flowerornament]

Summary

Closes #9.

This fixes two production code-splitting edge cases:

• alias-resolved modules outside the entry root can be assigned collector labels such as _external/*; chunk output selection was matching by path suffix, so those modules could be omitted from the emitted chunk;
• dynamic CSS imports such as import("./theme.css") should not survive as browser-runtime imports of CSS/data modules when Volt already owns stylesheet output.

The patch keeps Volt's existing chunk-graph approach, but makes three targeted changes:

• select chunk files through the collector's original module path to label map;
• protect dynamic import(...) before per-chunk OXC bundling, then restore it before Volt rewrites chunk URLs;
• rewrite dynamic CSS import expressions to an already-resolved no-op promise. Static CSS imports continue to use the existing no-op JS-module path.

Repro

The public issue has the reduced cases and expected output: #9.

This PR adds tests for both cases:

• code splitting includes alias modules outside the entry root
• dynamic CSS imports become inert browser-loadable modules

Validation

• mix format --check-formatted
• mix test test/builder_test.exs --seed 0

AI disclosure

I used OpenAI Codex as a coding assistant to help reduce the repros and draft the candidate patch. I reviewed the diff and ran the validations above locally.

[flowerornament]

This resolves my specific issue, but you may want to review for generality