feat(test): write down the safety invariants and test them as invariants

[mkoushni]

Summary

• Write the fail-closed promise down as testable claims in docs/safety-invariants.md (linked from CONTRIBUTING) and drive them with a reusable fault-injection plugin and PDP resolver.
• Contain panics in serial, transform, and audit the same way concurrent already did: route through on_error instead of unwinding execute(). Halt vs continue still follows can_block; that difference is documented with a reason.
• Table-driven catalog: every plugin phase × {panic, error, timeout} under on_error: fail, plus ignore/disable for serial, transform, and audit. A sequential panic under ignore still runs later audit; under fail it does not.
• Malformed config covers unknown key, misspelled top-level block, and unparseable YAML. Omitted on_error in YAML is Fail. Missing subject.id pins the differential allowlist row (Cedar dispatch error, CEL eval error, OPA default deny). Malformed policy pins Cedar dispatch / CEL compile / OPA compile-or-dispatch.
• PDP {panic, error, timeout} cells drive the evaluator wrapper (FaultPdp), not the Cedar/CEL/OPA resolvers themselves. Real dialects are covered for missing attribute and malformed policy. A new phase or shipped dialect fails the build until a cell is added.

Closes #24

Test plan

• cargo test -p praxis-policy-core --test safety_invariants
• cargo test -p praxis-policy-apl-core --test safety_invariants
• cargo test -p praxis-policy-pdp-diff safety

[praxis-bot]

Review: feat(test): write down the safety invariants and test them as invariants

Strong work. The containment model is sound: serial, transform, and audit panics now route through on_error the same way concurrent already did. The catalog approach -- exhaustive matches that fail the build when a phase or dialect is added -- is exactly the right mechanism for these invariants. The fault-injection infrastructure is well-factored and the test matrix is thorough.

Two findings, both medium.

Severity	File	Finding
Medium	executor.rs	ContainedOutcome::Lost non-blocking paths skip OnError::Disable
Medium	plugin.rs	is_dispatch_phase takes self by value; all other PluginMode methods take &self