refactor: collapse the two projects into one tree and move to npm

The repo held two near-duplicate projects. 2016/ and 2017/ each carried their
own React frontend, their own copy of the same Rust crate, and their own
package manager setup. 2016/tools/sync-from-thptqg2017.sh existed purely to
copy the parser source between them.

New layout:

  index.html + src/  the 2017 frontend, now the only one
  data/<id>/         2016, 2017, 2017-old, 2017-old2
  parser/            the single Rust crate, configs renamed to <id>.toml
  docs/              both projects' docs, 2016 copies suffixed -2016-legacy
                     pending the merge pass

<id> is now one identifier end to end: data/<id>/ feeds parser/configs/<id>.toml
and produces db/<id>.db.gz.

pnpm gives way to npm. pnpm-workspace.yaml existed only to whitelist
better-sqlite3's native build, which npm permits by default, so it has no
equivalent and is simply gone. Lockfiles cannot be converted; package-lock.json
is generated fresh. The migration direction is safe — pnpm's strict layout
forbids phantom dependencies, so anything that resolved under pnpm resolves
under npm's flat tree.

Adds parser/scripts/build-db.js and src/datasets.js: the four dataset IDs are
declared once and read by both the build tooling and (from the next phase) the
frontend.

Follow-on fixes the move made necessary:
  - eslint's Node-globals override pointed at scripts/, now parser/scripts/
  - crawl-baotintuc.js wrote to <root>/data, now data/2017
  - golden tests loaded configs by their old thptqg*-data.toml names

Drops the #[ignore]d Rust-vs-Node golden test. It shelled out to pnpm to run
scripts/build-database.js, a file removed when the parser was ported to Rust,
so it could never pass. check-duplicates.js and diff-datasets.js were already
broken before this change and are annotated as such rather than half-fixed.

63 Rust tests pass and clippy is clean from the new location.

docs/README.md

@@ -0,0 +1,5 @@
+# Docs
+
+- [`system-architecture.md`](./system-architecture.md) — data flow, schema, 3-variant deploy, score-tier model, admission-block catalog
+- [`data-pipeline.md`](./data-pipeline.md) — Excel parse quirks, overflow-sheet gotcha, audit scripts, refresh flow
+- [`deployment-guide.md`](./deployment-guide.md) — GitHub Pages workflow, adding a new variant, rollback