v0.1.0-alpha.5

Fixes a publish-time regression in 0.1.0-alpha.3 (and the unreleased
alpha.4) where the package shipped raw TypeScript under main and
exports. Node 24 refuses to strip types from anything under
node_modules, so any consumer following the quickstart hit
ERR_UNSUPPORTED_NODE_MODULES_TYPE_STRIPPING on the first import
and could not use the library at all.

Changed

• npm run build compiles src/ to dist/ via tsconfig.build.json;
  prepublishOnly runs it automatically so the npm tarball always
  contains compiled JS plus declaration maps.
• main, types and exports."." now point at ./dist/index.js and
  ./dist/index.d.ts. The files array ships dist/ instead of
  src/, so consumers no longer see raw TS in node_modules.
• Dev shell and engines.node move from Node 22 to Node 24 (the
  current Active LTS) to match CI.

End-to-end verified against witness-demo.tulpa.network: the
quickstart submit.mjs now returns a signed inclusion receipt on
Node 24 without modification.