build: duplicate entry ids are not detected by the build itself; the second page silently overwrites the first #81

Why it matters
validate.mjs catches duplicate ids, so CI is safe, but npm run build alone does not. loadEntries keeps both entries and a Map keyed by id keeps the last body, so writeEntryPages writes dist/entry/<id>/index.html twice and the surviving page mixes one entry's metadata with the other's body. A contributor who previews with npm run build sees a plausible page and never learns there is a clash until CI fails, or worse, if someone deploys a local build.

Evidence

• site/build.mjs:75-77 (loadEntries, bodies.set(id, …)), :472-474 (writeEntryPages); no uniqueness check in between.

Suggested approach
Track ids in a Set inside loadEntries and push duplicate id "<id>" (<file1>, <file2>) to errors, which already aborts the build. Add a unit test once the helper is exported (see the test-coverage issue).

Done when
node site/build.mjs fails with a named error on a duplicate id.