QA: Arena v2 invariants, E2E suite, manual QA checklists

Context

Testing docs, invariants, and E2E for TimeArena v2. Parent epic: #238 (closed).

Relevant files

• docs/testing/invariants-and-business-logic.md
• docs/testing/manual-qa-checklists.md
• docs/testing/e2e-anvil.md
• .cursor/skills/yieldomega-guardrails/SKILL.md

Acceptance criteria

• New INV-TIME-ARENA-* invariant IDs for timers, CRED, prizes, XP, WarBow epochs
• Retire or archive TimeCurve invariant sections
• E2E specs renamed e2e/anvil-arena-*.spec.ts; pay-mode testids updated
• Manual QA checklist for multi-timer, CRED claim, wallet modal

Verification checklist

• CI green: contracts + indexer + frontend tests
• Maintainer review of spec ↔️ test map