test: add phase 2 dogfood gate

[aram-devdocs]

Summary\n- add a dogfood phase-gate test that runs all 8 MVP built-in rules together and asserts exact per-rule finding counts\n- gate CI preflight on cargo xtask pre-release so runbook/schema/rules-index checks are covered in PR CI\n\n## Local validation\n- git diff --check\n- Rust toolchain unavailable in this runtime (cargo not found); CI is the source of truth

[github-actions]

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PR #138 — test: add phase 2 dogfood gate

Files: crates/plumb-core/tests/dogfood_phase_gate.rs (+316), .github/workflows/ci.yml (+2), crates/plumb-mcp/AGENTS.md (-1 blank line)

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1. Determinism

BTreeMap appears only in the test-local fold over violations — not in any output path. All fixture data is static slices with deterministic insertion order. IndexMap is correctly used for ColorSpec::tokens matching the type definition. The run() function's sort guarantee ((rule_id, viewport, selector, dom_order)) means the into_iter().fold() receives violations in a stable order regardless of how the rules fire internally. Clean.

2. Workspace layering

dogfood_phase_gate.rs lives under crates/plumb-core/tests/ and imports exclusively from plumb_core. No cross-crate deps, no layering violation. Clean.

3. Error handling

No library code changed. clippy.toml has allow-unwrap-in-tests = true / allow-expect-in-tests = true, so the .or_default() fold and assert_eq! pattern are fine under the workspace lint config. Clean.

4. Test coverage

The dogfood gate is well-structured:

• Covers exactly the 8 rules returned by register_builtin() — nothing missed, nothing invented.
• assert_eq!(by_rule, expected) on a BTreeMap catches both missing rules and unexpected extra rules in one assertion.
• Clean-control nodes (#controls) guard against cross-rule noise — good defensive design.
• #[allow(clippy::too_many_lines)] on dogfood_snapshot() is correctly required: pedantic is set to warn workspace-wide and RUSTFLAGS: -Dwarnings promotes it to error in CI.

5. CI change — cargo xtask pre-release in preflight

sync_rules_index() is read-only (validates docs pages exist, bails if any are missing; does not write). pre_release() therefore fails fast on schema drift or undocumented rules without mutating the workspace. Correct placement in preflight — which all downstream jobs depend on — ensures these checks gate the full matrix.

Nit: .github/workflows/ci.yml:20 — the job name still reads Preflight (fmt, clippy, check, agents-md); xtask pre-release is not reflected. Not a blocker, but the name will mislead future readers.

6. Documentation

No new public items added. No docs/src/** changes. Humanizer check not required.

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Punch list

• .github/workflows/ci.yml:20 — job name should mention xtask pre-release (nit; won't fail CI).

Verdict: APPROVE