Add generic record casts rule

[benvinegar]

Summary

• add a new types.generic-record-casts rule for as Record<string, unknown> casts on vague parsed/payload variables
• register the rule in the default pack and document it in the main README plus a dedicated rule README
• add focused tests and heuristic-regression coverage for the new rule

Benchmark signal

• full pinned benchmark signal score: 0.7777777777777778
• best separating metric: findings / file (0.7777777777777778)
• hit rate: 5/9 AI repos vs 0/9 mature OSS repos

Validation

• bun test tests/generic-record-casts.test.ts tests/heuristics.test.ts tests/fixtures-regression.test.ts tests/benchmark.test.ts
• bun run lint

This PR description was generated by Pi using GPT-5