Support string and conditional package.json exports

[laat]

Summary

• Resolve the module main entry through a new resolveMainEntry helper that handles every common shape of package.json exports:
  • string ("exports": "./lib/main.js")
  • subpath map ("exports": { ".": "./lib/main.js" })
  • bare conditional ("exports": { "import": "./esm.js", "require": "./cjs.js" })
  • subpath + conditional nesting ("exports": { ".": { "import": { "default": "./esm.js" } } })
• Preserves the existing options.main > pkg.main > pkg.exports > "./index.js" precedence.

Why

src/run.js used pkg.exports?.["."] to pull the root export, which silently returns undefined for the string form and for bare conditional maps, falling through to ./index.js. This package's own package.json has "exports": "./src/index.js", so any README that imports readme-assert by name would be rewritten to a nonexistent path. Reproduced with a fixture pkg whose exports is ./lib/main.js — before: import rewritten to .../index.js; after: .../lib/main.js.

Test plan

• New resolveMainEntry unit tests cover string / subpath / conditional / nested / null cases (9 cases).
• New integration fixture test/fixtures/pkg-string-exports/ exercises the full processMarkdown + run path through an ESM package with "exports": "./lib/main.js".
• node --test test/*.test.js — 55 tests pass (11 new).
• pnpm -r test — all 10 workspace example packages still pass.