Fix SDK runtime subpath bundles

[notmike101]

Summary

• build each public SDK entrypoint as a self-contained ESM and CommonJS bundle
• make the main and renderer runtime subpaths executable instead of bare undefined re-exports
• exercise every subpath in Bun and Node, then bump host and SDK to 0.3.1

Verification

• bun audit — no vulnerabilities
• bun install --frozen-lockfile — no changes
• bun run check — 30 tests, SDK type consumer, Node ESM/CommonJS subpaths, example, native host build
• isolated npm tarball consumer — Node ESM and CommonJS root/main/renderer/experimental imports passed
• npm pack ./sdk --dry-run --json — 28 files, SHA-1 ff6536082a3d03e59118316034d08f932c75ba8c
• release metadata verified for v0.3.1