Compile-safety regression sentinel only checks method names, not behavior

Found during an independent pre-merge review of #79 (1.1.0).

CompileSafetyValidatorSentinelTest (added in 1.1.0 to guard against reintroducing per-module A2-style validation) only reflects over method names — it asserts validate/flushDeferred are absent and validateFullGraph is present. A semantically-equivalent reintroduction of per-module validation logic under a different method name, or inlined directly into validateFullGraph, would pass this sentinel undetected.

The real protection today is the behavioral regression tests (e.g. cross_module_peer_provider_ok.kt), which are sound on their own. This sentinel is closer to documentation-as-test than an actual guard.

Possible directions (not decided): a behavioral sentinel that asserts a genuine per-module false-positive shape (like the :core:notifications/PeerService case that motivated the 1.1.0 removal) stays green regardless of internal refactoring, rather than asserting anything about method names.

Scope: 1.1.1 or 1.2.0, not blocking 1.1.0.

Start with CompileSafetyValidatorSentinelTest and the behavioral regression coverage around cross_module_peer_provider_ok.kt. Review the :core:notifications/PeerService case that motivated the 1.1.0 change, then determine how to make the sentinel behavioral rather than method-name based. Done means the genuine per-module false-positive shape remains green despite internal refactoring.