fix(serve): variant discovery surfaces parse errors + reads wrapped variant files (REQ-260, REQ-262)

[avrabe]

What (v0.28 slice 1 — variant P1 silent-drops, DD-076)

Two silent-drops in ProjectVariants::discover:

• REQ-260 — the .ok() sites swallowed parse failures: a broken feature-model.yaml became model: None and resolve() reported the misleading "no feature model configured"; a broken bindings.yaml rendered artifact_count: 0 silently. discover now captures failures into model_error + diagnostics, logs them, and the /variants page shows a red "Feature model failed to load" banner instead of "no variants configured". resolve() distinguishes model-present-but-broken (returns the parse error) from model-absent (friendly hint).
• REQ-262 — variant files were parsed with raw serde_yaml::from_str (flat only), so an init-scaffolded variant:-wrapped file was silently invisible in serve (the variant: rivet variant init emits a binding format that rivet variant check rejects #514 regression on the serve path). Now uses VariantConfig::from_yaml_str (both shapes); model loading uses load() so a composed feature-model-binding file isn't dropped.

Verification

• 4 new tests: discover_broken_model_records_diagnostic_and_resolve_reports_parse_error, discover_absent_model_still_reports_absent_message, discover_accepts_wrapped_variant_shape, discover_flat_variant_still_works — all pass (11 serve::variant + 8 variant_scoped_api).
• Build clean; cargo clippy --all-targets -- -D warnings on Rust 1.97 clean; fresh-binary rivet validate PASS.

Implements: REQ-260, REQ-262 · Refs: DD-076

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[github-actions]

📐 Rivet artifact delta

Change	Count
Added	0
Removed	0
Modified	2
Downstream impacted (depth ≤ 5)	0

Graph

graph LR
  REQ_260["REQ-260"]:::modified
  REQ_262["REQ-262"]:::modified
  classDef added fill:#d4edda,stroke:#28a745,color:#155724
  classDef removed fill:#f8d7da,stroke:#dc3545,color:#721c24
  classDef modified fill:#fff3cd,stroke:#ffc107,color:#856404
  classDef overflow fill:#e2e3e5,stroke:#6c757d,color:#495057,stroke-dasharray: 3 3

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Modified

ID	Changes
REQ-260
REQ-262

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