chore: expose experimental preprocessor subpath export

[rlippmann]

Summary

Adds package export wiring for the experimental preprocessor subpath and updates imports to use the public experimental entry point.

Changes include:

• package export for @rlippmann/context-compiler/experimental/preprocessor
• preprocessor tests updated to use the public subpath
• Node example updated to use the public experimental import
• docs updated for the experimental import path

Why

The preprocessor is experimental and should remain outside the root package surface, but consumers need a stable public import path for the experimental module.

Scope checklist

• Diff is minimal and scoped to the requested behavior
• Python/spec parity is preserved (or mismatch is explicitly documented)
• Fixture semantics are respected (exact string vs non-empty null rule)
• No unrelated refactors
• Relevant tests were run

Notes

Root package export remains core-only.

The subpath currently points to dist/src/experimental/preprocessor/... because of the existing build layout. A cleaner emitted layout can be handled later as packaging cleanup.

Vitest runs against source, so it needs a test-time alias for the experimental package subpath. Published package behavior still comes from the package.json export map.

Validation:

• npm test
• npm run fixtures:check
• npm run fixtures:preprocessor:check
• npm pack