fix: rewrite local declare module specifiers to output chunk paths

[igordanchenko]

• This PR contains AI-generated code, but I have carefully reviewed it myself. Otherwise, my PR may be closed.
  • I understand that my PR is more likely to be rejected or requested for changes if it contains AI-generated code that I do not fully understand.

Description

When declare module './foo.js' or declare module '@/foo.js' references a local file, the specifier was preserved as-is in the bundled .d.ts output. This made the augmentation unreachable for consumers since the internal path no longer resolves after bundling.

Now resolves local declare module targets and rewrites their specifiers to the relative path of the output chunk containing the target module.

How it works

1. transform - resolves all declare module string literal specifiers using this.resolve(). If the target is a local (non-external) module, stores the resolved module ID in the declaration metadata.
2. renderChunk - replaces resolved specifiers with an internal placeholder.
3. generateBundle - builds a module-to-chunk mapping and rewrites placeholders to the correct relative path between output chunks.

This approach works for both same-chunk (self-referencing) and cross-chunk augmentations, as well as tsconfig path aliases like @/foo.js.

Test plan

• New test case with cross-chunk rewrites, subfolder resolution, and same-chunk self-references
• Existing declare module test (ambient declare module 'virtual') still passes unchanged
• Full test suite (194 tests), lint, typecheck all pass

Linked Issues

Fixes #134

Additional context

While the original ticket #134 mentions same-chunk references being replaced with declare module '.', I couldn't get that working with TypeScript. This PR rewrites same-chunk specifiers as declare module './current-chunk.js' instead.