test: add proactive system integrity contracts

Replaces reactive bug-specific tests with 4 structural CONTRACTS
that make entire categories of bugs impossible:

1. Serialization Contract (13 parametrized hostile payloads):
   Proves SessionStateEncoder NEVER crashes regardless of what
   gets injected onto args (datetime, lambda, bytes, inf, nan, etc.)

2. Persistence Contract:
   Forces the REAL persist() path by removing PYTEST_CURRENT_TEST guard.
   Validates write → read round-trip with poisoned sessions.

3. Production Parity Contract:
   Scans ALL GramAddict source for pytest/test-mode divergence guards.
   Any unaudited divergence point fails the build. New guards require
   explicit justification in KNOWN_DIVERGENCES registry.

4. Import Integrity Contract:
   Imports every single GramAddict module to catch syntax errors,
   circular imports, and missing dependencies at test time.

This addresses the root systemic failure: tests were running in a
parallel universe where serialization was silently skipped, allowing
a datetime injection to corrupt sessions.json and kill production.

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