feat: impl catalog schema v2

[aceforeverd]

Schemas

• variant.schema.json (new): one variant per file, with apiVersion: catalog.swiss/v2 and kind: Variant. version (semver) and releaseDate are required, plus optional deprecated and replaces.
• metadata.v2.schema.json (new): adds defaultVariant, series and quantization. Tuning refs must be @, and the legacy version stays optional for index.json.
• index-v2.schema.json (new): v2 models and their variant files only.
• catalog.schema.json: adds repo and raw; catalog.yaml sets repo.

Library and build

• hack/lib/history.js (new): one walk of master's first-parent git history, with the bytes of each change. A shallow clone is refused.
• hack/lib/catalog.js:
  • Files are classified by content (versionFiles, variantFiles).
  • buildIndexV2() builds index-v2.json and every history.json.
  • checkIndexV2() checks the result against its schema and history.
  • revisionProblems() enforces that every change raises the version and changes something else.
  • buildIndex() publishes a v2 model's legacy tuning with ids stripped, so index.json is unchanged.
• server/data.js and server/index.js:
  • The build and the catalog server also write and serve index-v2.json, the v2 variant files and the histories.
  • catalog.json reads tuning ids without @Version, so the site renders exactly as before.

Validators

• validate:schema:
  • checks v1 and v2 files by content;
  • checks the v2 path and id, LWS size against nodes, and that the chart matches the engine;
  • requires defaultVariant to name a real variant and tuning to pin an existing version;
  • applies the version rule and refuses a committed history.json;
  • with --base, also refuses deleting a variant that wasn't deprecated, and new v1 files in a v2 model.
• validate:helm and validate:images cover variant files. The image check reads a tuning ref's pinned version, from git when it isn't the current one.
• hack/migrate-v2.js (new): migrates a model from its latest v1 file, with a --dry-run diff. It refuses any variant whose content would change.

CI and repo

• Both workflows use fetch-depth: 0, and lint runs validate:schema --base .
• The pre-commit hook runs validate:schema --base HEAD.
• .gitignore ignores *.history.json under models/.
• package.json adds the migration test to npm test.
• README.md gains a short Schema v2 spec.

Tests: fixture-repo tests for the library, server tests for index-v2 and the catalog.json pairing, a migration test over every v1 model, and Helm selection tests for variant files. 70 node and 11 site tests pass.