experiment: replace @effect/docgen with TypeDoc for clickable cross-referenced module docs

[solidsnakedev]

Context

The current module reference docs are generated by `@effect/docgen@0.5.2` using the `mikearnaldi/docgen-template` theme. Output is static MDX with signatures and JSDoc text — no clickable type cross-references, no source links to GitHub.

The goal is Haddock-style behaviour: every type name in a signature is a link that jumps to that type's own documentation page.

Proposed Experiment

Replace `@effect/docgen` with TypeDoc + `typedoc-plugin-markdown` on an isolated output path so both pipelines can run side-by-side without breaking the existing `/docs/modules/` route.

What TypeDoc gives us

• Clickable cross-references for every type in every signature (uses the TypeScript compiler API to resolve them)
• Source links to GitHub out of the box via `sourceLinkTemplate`
• Handles generics, branded types, class methods, and conditional types correctly
• `typedoc-plugin-markdown` emits MDX consumable by fumadocs

Implementation plan

1. Add `typedoc`, `typedoc-plugin-markdown`, and `typedoc-plugin-frontmatter` as dev dependencies in `packages/evolution`
2. Add a `typedoc.json` config in `packages/evolution` pointed at existing `src/` — output to a new `docs-typedoc/` directory
3. Add a `generate-typedoc-docs` script in `docs/scripts/` that copies the output into `docs/content/docs/api/` (parallel to the existing `modules/` path)
4. Wire the new path into fumadocs `source.config.ts` under a separate `/docs/api/` route
5. Audit the output: verify cross-references resolve, source links point correctly, category grouping is preserved
6. Compare UX against the existing `/docs/modules/` output and decide whether to migrate fully or keep both

Acceptance Criteria

• TypeDoc generates MDX without errors for all exported symbols in `packages/evolution/src`
• Cross-reference links (e.g. clicking `Data.Constr` in a TSchema signature) resolve to the correct page
• Source links open the correct line in GitHub
• The new output is accessible at `/docs/api/` in the docs site without affecting `/docs/modules/`
• Existing `@effect/docgen` pipeline is untouched

Notes

• This is explicitly experimental — full migration is a separate decision pending the audit
• `externalPattern` exclusions for Effect library internals will likely need tuning to avoid polluting the output with Effect framework types