Add the breadth-follows-evidence rule to CONTRIBUTING.md

> Written to the standard set by #33 — read that issue first for the expected level of detail. ## What Add the stated rule to CONTRIBUTING.md or docs/checks.md, wherever the existing maintainer-owned-areas language lives: > **Breadth follows evidence.** A corridor, metric, or score is added only when its underlying observation can be reproduced, evidenced, and evaluated under the project's measurement contract. This is the project's answer to the question "how do you decide what to measure?" — and the answer is: only what can be observed and verified, never what would look impressive on a dashboard. ## Why it matters Without this rule stated explicitly, a contributor could add a corridor that sounds plausible (e.g., "USDC → KES" without verifying a real issuer exists) or a metric that sounds useful (e.g., "liquidity score" without defining what the components are). The rule sets the bar: no observation, no addition. This is the same standard that already governs the measurement contract (every figure must come from a live source or recorded snapshot), but applied to the *scope* of what gets built, not just the *fidelity* of what gets reported. ## Which layer **Layer 4 — documentation only.** No code. Process rule. ## Exactly where the code goes - **Modified file:** `CONTRIBUTING.md` — in the "Measurement discipline" section (around line 40) - Or alternatively: `docs/checks.md` — in the "Constraints" section (around line 180) - One sentence, added as a bullet or a standalone paragraph ## Where the data comes from No data. This is an editorial addition to an existing document. The rule originates from the project's design philosophy: everything Wayfare publishes is a reproducible measurement, and the scope of what it measures should be held to the same standard as the measurements themselves. ## What "unmeasurable" means here Nothing is measured. This is a docs-only change. ## Acceptance criteria - [ ] The rule is stated in CONTRIBUTING.md or docs/checks.md (maintainer decides which) - [ ] The rule uses the exact phrasing: "Breadth follows evidence" - [ ] The rule is placed near the existing measurement discipline or constraints language - [ ] No code changes — docs only - [ ] The prose matches the surrounding voice ## Out of scope Do not change: - Any Go code - The issue template or PR template - The README (this is a contributor-facing rule, not a user-facing one) ## How to verify locally ```bash grep -n "Breadth follows evidence" CONTRIBUTING.md docs/checks.md ``` No build or test needed.