docs(rules): add escalation testing and a spike protocol to testing.md

Two additions. An "Escalation Beyond Category and Pairwise" section adds property-based testing (for invariants across a broad input domain) and mutation testing (for when high coverage hides thin assertions), both as escalation paths rather than always-on gates. And the "I need to spike first" excuse is formalized into a disciplined spike protocol: TDD stays the default, but a spike is sanctioned only when timeboxed, not committed, and followed by the first failing test before productionizing.

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