test: mutation-harden protection unit coverage

[thedavidmeister]

Module hardened

src/order/protection.ts — downscaleProtection, the owner spam-protection that reduces each owner's per-round order limit by comparing the owner's average vault balance of a token against the rest of the orderbook's balance of that token. It was the weakest-covered qualifying module in src/order/** (branch coverage 68.75%, only 3 tests for 100 lines of dense limit-divide arithmetic and owner/token/vault grouping).

This PR is tests-only — no change to src/order/protection.ts (verified git diff src/order/protection.ts empty). It adds a downscaleProtection limit-divide arithmetic block with 13 discriminating cases that pin each intermediate of the calculation to an exact value.

Mutation matrix

Each behavior's source line was mutated and the protection suite re-run. The pre-existing 3 tests left 9 of 13 mutants alive; the new tests kill all 13.

#	Mutation (behavior)	Pre-existing 3 tests	With new tests
1	Math.max(round(limit/avgCut), 1) → drop max (min-1 clamp)	KILLED	KILLED
2	Math.round → Math.trunc (rounding)	KILLED	KILLED
3	cuts.reduce(+)/cuts.length → drop /length (per-token cut averaging)	SURVIVED	KILLED
4	reduce((a,b)=>a+b) → (a,b)=>a (cut sum)	KILLED	KILLED
5	ownerTotal / BigInt(vaults.length) → drop divide (per-vault avg)	SURVIVED	KILLED
6	obTokenBalance - ownerTotal → + (other-owners balance)	SURVIVED	KILLED
7	=== 0n ? 100n → ? 1n (zero-balance ratio short-circuit)	SURVIVED	KILLED
8	avgBalance * 100n → * 50n (ratio percent scaling)	SURVIVED	KILLED
9	Math.min(., 3) → min(., 2) (segment cap)	SURVIVED	KILLED
10	- 4 → - 5 (segment offset)	KILLED	KILLED
11	/ 25n → / 20n (segment width)	SURVIVED	KILLED
12	admin-limit continue → disabled (skip admin-set owners)	SURVIVED	KILLED
13	obTokenBalance === undefined) continue → disabled (skip failed read)	SURVIVED	KILLED

Gaps checklist (behaviors now pinned)

• 4-segment divide factor across every band: [0,25)→4, [25,50)→3, [50,75)→2, [75,100)→1, ≥100→1 (parametrized)
• zero other-owner balance short-circuits ratio to 100% → factor 1 → limit unchanged
• per-vault balance averaging (total/count, not sum) before the ratio
• per-token divide-factor averaging (avgCut = sum(cuts)/count)
• max(round(limit/avgCut), 1) — both the rounding (2.5→3, not 2) and the min-of-1 clamp
• admin-configured owners skipped entirely (no readContract, limit untouched)
• token whose orderbook balance read fails is skipped (contributes no cut)
• orderbook token balance cached per (orderbook, token) — one readContract for two vaults of the same token

Verification

• tsc -p tsconfig.check.json clean; eslint src/order/protection.test.ts clean.
• Order suite: 8 files, 92 tests pass (was 79; +13).
• Full vitest unit suite: 59 files, 854 tests pass.
• The e2e fork test jobs are a pre-existing environmental red (RPC/secrets), unrelated to this change; no e2e/fork tests touched.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

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Run ID: 085bf8f8-8683-4257-8260-8df961f166b3

📥 Commits

Reviewing files that changed from the base of the PR and between 4928590 and f44859d.

📒 Files selected for processing (1)

• src/order/protection.test.ts

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch 2026-06-15-solver-order-coverage

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