feat: set up rivet SDLC artifact traceability

[avrabe]

Summary

• Adds rivet.yaml project configuration with common, stpa, aspice, dev schemas
• Converts REQUIREMENTS.md into 22 rivet artifacts (BR, FR, NFR, TR) with ASPICE V-model types
• Adds 6 architecture component artifacts (ARCH-001 through ARCH-006) with requirement traceability
• Adds 6 verification artifacts (VER-001 through VER-006) linking proofs/tests to requirements
• Wires existing STPA safety analysis (losses.yaml, hazards.yaml) via stpa-yaml adapter
• Configures commit traceability (Implements/Fixes/Verifies trailers)
• Cross-repo externals for kiln and meld

53 total artifacts, rivet validate passes with 24 warnings (expected for initial setup — missing STPA steps 2-4, NFRs lacking verification backlinks).

Type	Count	IDs
stakeholder-req	4	BR-001 to BR-004
system-req	13	FR-001 to FR-008, NFR-001 to NFR-005
sw-req	5	TR-001 to TR-005
system-arch-component	6	ARCH-001 to ARCH-006
sys-verification	6	VER-001 to VER-006
loss	6	L-1 to L-6
hazard	10	H-1 to H-10

Test plan

• rivet validate passes (PASS, 24 warnings)
• rivet stats shows 53 artifacts
• CI passes (no Rust changes)

🤖 Generated with Claude Code