feat(spar-sysml2): roundtrip generation — rivet YAML to SysML v2 (Phase 4 of #90)

[avrabe]

Summary

Phase 4 of #90 — completes the SysML v2 ↔ rivet roundtrip.

```
SysML v2 → spar extract → rivet YAML → spar generate --from-rivet → SysML v2
```

CLI

```bash

Generate SysML v2 from rivet artifacts

spar generate --from-rivet artifacts/requirements.yaml -o requirements.sysml

Also available as spar sysml2 generate

spar sysml2 generate --from-rivet artifacts/sysml-extracted.yaml
```

Generated output

• `requirement def` with doc strings for each requirement
• `part def` / `action def` / `state def` for tagged components
• All 5 relationship types: satisfy, verify, refine, allocate, derive

New API

• `spar_sysml2::generate::parse_rivet_yaml(yaml) → Vec`
• `spar_sysml2::generate::generate_sysml2(artifacts) → String`

Test plan

• 12 tests (YAML parsing, generation, all relationship types, roundtrip)
• Clippy clean, formatted
• CI passes

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 82.89157% with 71 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
crates/spar-cli/src/main.rs	3.57%	54 Missing ⚠️
crates/spar-sysml2/src/generate.rs	95.26%	17 Missing ⚠️

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