feat(coverage): let a project declare rules it does not model (REQ-320)

[avrabe]

Closes the remaining half of #871.

Adopting an embedded preset for part of its scope drags in the rest of its
rules: taking aspice for SWE.1/SWE.6 necessarily inherits SWE.2/3/4 rows the
project has no intention of satisfying. There was nowhere to say so, and an
unmodelled level rendered as an empty row forever — indistinguishable from
one nobody got round to.

coverage:
  unmodelled-rules:
    - rule: swe3-refines-swe2
      reason: detail design lives in a parallel dev spine, by intent

The reason is required. A declaration without one is indistinguishable from
suppressing an inconvenient row, which is the thing this must not become. And
the declared rule stays in the report, annotated — hiding it is the failure
this replaces, not the fix.

Two ways a declaration fails, and both fail the run

stale	the rule now has source artifacts — the project said it doesn't model this, then modelled it
unknown	no such rule in the active schemas — silently ignoring a typo lets a project believe it declared something

The staleness half is the point, and it's what I promised you when this was
filed: an exemption that outlives its reason is the same defect as the 100% it
replaced — a number that stopped meaning what it says. It fails always rather
than under a flag, because a stale exemption silently shrinks the denominator
every later number is computed against.

Grouped, not standalone

Deliberately built alongside REQ-309 (exempt-when-field — an artifact declares
itself out of a rule's population) and REQ-313 (declared-blocked acceptance
criteria). Three levels of one idea — artifact, criterion, project scope —
and this is the project-scope one. Three unrelated mechanisms would each have
grown their own staleness check and given a reader three places to look when a
number goes quiet.

Notes on the build

• Implemented additively as mark_unmodelled rather than a parameter on
  compute_coverage, whose signature is public.
• The oracle was rewritten against real dev-schema rule names after a first
  draft guessed one that doesn't exist — a fixture naming a rule the schema has
  never heard of exercises nothing. decision-justification (empty population)
  is the happy path; requirement-verification (populated) is the stale path.
• Unit tests live in rivet-core beside the function because the mutation gate
  runs -- --lib. Negative-controlled there: never detecting stale, ignoring an
  unknown rule, and not annotating each redden exactly one test.
• rivet-core/src/proofs.rs needed the new field too. That file is
  #[cfg(kani)] — invisible to build, test and clippy. The same omission broke
  the Kani proofs once before and the signal was written off as flake six times.
  Verified structurally (14/14 fields) since --cfg kani can't build locally: a
  dependency carries its own kani-gated code.

Heads-up: this adds public fields to ProjectConfig and CoverageEntry, so
Semver Checks may flag them. Precedent says fields have been added before
(release, externals, baselines) — I'd rather let CI answer than predict.

Gates

cargo fmt --check · clippy --all-targets -D warnings on 1.97.0 ·
cargo test --workspace (exit 0, 66 ok) · full cli_commands (189 passed) ·
rivet validate · rivet docs check · yamllint · rivet coverage on this
repo (unchanged — no declaration means no behaviour change) — all exit 0.

With this, every artifact scoped to v0.36.0 is verified (10/10).

[github-actions]

📐 Rivet artifact delta

Change	Count
Added	0
Removed	0
Modified	1
Downstream impacted (depth ≤ 5)	0

Graph

graph LR
  REQ_320["REQ-320"]:::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

Loading

Modified

ID	Changes
REQ-320

📎 Full HTML dashboard attached as workflow artifact rivet-delta-pr-893 — 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.

📢 Thoughts on this report? Let us know!