kernel: align VERSION with the beta target (0.0.1.dev0) and document the versioning scheme

[JArmandoAnaya]

VERSION was bootstrapped at 0.1.0.dev0, which sorts above the 0.0.1-beta release
target and therefore blocks tagging any v0.0.1-alpha.N. This drops it to 0.0.1.dev0
and resyncs everything derived from it.

Changes

• VERSION → 0.0.1.dev0; frontend packages resynced to 0.0.1-dev.0; openapi.json
  regenerated (its info.version is derived from VERSION, so it drifts on every bump).
• scripts/sync_versions.mjs: the PEP 440 → npm mapping now covers a/b/rc as well as
  .devN, so the documented beta mapping (0.0.1b1 → 0.0.1-beta.1) actually works.
  Unmappable versions throw instead of being written verbatim into a package.json.
• New pnpm version:check (--check mode), wired into CI as a drift gate so a VERSION
  bump without pnpm version:sync fails the build.
• Tests: tests/scripts/sync_versions.test.mjs (node:test) covers the mapping;
  tests/test_versioning.py covers VERSION → installed distribution → visionset --version.
• CONTRIBUTING: full versioning section — the PEP 440 ↔ npm table, and the tag-vs-publish
  scheme (v0.0.1-alpha.N are git tags only, never published; the beta is 0.0.1b1 on
  PyPI / 0.0.1-beta.1 on npm, lockstep across the monorepo).

Gates

uv run pytest (29 passed, incl. 3 new) · uv run lint-imports (2 contracts kept) ·
uv run mypy src/visionset/kernel + src/visionset · uv run ruff check . /
ruff format --check · pnpm -r lint · pnpm -r build · pnpm test · pnpm version:check

Closes #2