docs(artifacts): file REQ-085 (cross-repo composition) + REQ-086 (witness MC/DC)

[avrabe]

Summary

Two artifact-only filings tracking follow-ons from the v0.12.0 (REQ-083)
multi-file feature model composition work. No code changes.

• REQ-085 — cross-repo feature model composition. Extends REQ-083
  to mount sub-models from external git repos: a mount's model:
  becomes either a local path or <external-prefix>:<path>, resolved
  against rivet.yaml's externals: (the single source of truth for
  external repos — no git config duplicated in the binding). Rides
  existing rivet sync plumbing. v0.13.0-track.
• REQ-086 — MC/DC coverage of the composition core via the
  pulseengine witness tool. Captures two findings: (1) native
  cargo llvm-cov --mcdc is dead-ended on rustc nightly toolchain skew
  (rolling nightly dropped mcdc; the dated nightly that accepts it
  no longer compiles current deps) — exactly the churn witness's
  "post-rustc Wasm" design argument calls out; (2) the composition core
  is mostly pure and compiles to wasm32 cleanly. Plans a small
  compose-witness Wasm component + a witness harness, emitting a
  signed MC/DC envelope Rivet ingests as REQ-083 requirement-to-test
  evidence — closing the witness→rivet loop the ecosystem was
  architected for. v0.13.0-track.

Test plan

• rivet validate PASSes; the only diagnostics on REQ-085/086
  are the standard "draft requirement not yet satisfied"
  coverage INFO.
• CI green (artifacts-only PR; no code paths exercised).

🤖 Generated with Claude Code

[github-actions]

📐 Rivet artifact delta

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

Graph

graph LR
  REQ_085["REQ-085"]:::added
  REQ_086["REQ-086"]:::added
  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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Added

• REQ-085
• REQ-086

📎 Full HTML dashboard attached as workflow artifact rivet-delta-pr-313 — download from the workflow run.

_{Posted by rivet-delta workflow. The graph shows only changed artifacts; open the HTML dashboard (above) for full context.}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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