feat(variant): Phase 2 — variant config loading + validate/coverage wiring

[avrabe]

Context

Phase 1 MVP shipped in #285: Artifact::fields_per_variant + fields_for_variant() Cow-based resolver, yaml_hir typed parser. The building block is in place; nothing consumes it yet across validate / coverage / serve.

Phase 2 scope

Per docs/design/variant-aware-properties.md:

1. Load variant configs from artifacts/variants/*.yaml into the schema/project context.
2. Add --variant <name> flag to relevant commands (validate, coverage, list, query).
3. Validate honours per-variant field overlays when checking required-fields / allowed-values per artifact type.
4. Coverage scopes counts by active variant where the variant alters delegated chains.
5. Serve / docs surface the active variant in the artifact view.

Out of scope (Phase 3+)

• Multi-variant cross-product reports.
• Variant-aware supplier anchors (when: clause on external-anchor).

Refs: design doc §6 Phase 2, original issue #255.

[avrabe]

PR #291 opened against this issue.

Scope status:

• Items 1, 2, 3 (variant config loading + --variant on
  validate/coverage/list/query + per-variant overlay validation):
  done with lib + integration tests, clean clippy + fmt.
• Item 4 (coverage delegated-chain scoping by variant): deferred —
  the current PR stamps the active variant into the coverage report
  output but does not rewrite the chains. Tracked as a follow-up.
• Item 5 (dashboard variant selector + override marker): deferred —
  pure UI work on top of Artifact::fields_for_variant, no core
  changes needed.

Both deferred items are called out in the PR body's "Deferred to
follow-up" section.

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