feat: feature model schema + constraint solver (PLE Phase 3)

[avrabe]

Summary

FODA-style feature model with propositional constraint solver and binding model.

• Feature tree with group types: mandatory, optional, alternative (XOR), or
• Boolean constraint propagation with fixpoint loop
• Ancestor propagation (child implies parent chain)
• Group validation (alternative = exactly 1, or = at least 1)
• Cross-tree constraints via s-expressions (implies, excludes)
• Binding model mapping features to artifact IDs + source globs
• YAML parsing with cycle detection and tree validation
• 12 unit tests covering propagation, violation detection, edge cases

Implements: REQ-042, REQ-043, REQ-044
Refs: FEAT-110, FEAT-111, FEAT-112

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 87.75934% with 59 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
rivet-core/src/feature_model.rs	87.75%	59 Missing ⚠️

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