Test that the PhiSQL specification examples round-trip without loss

[jzonthemtn]

Nothing in the test suite exercises the PhiSQL specification examples, so a field the engine does not read is ignored silently. A round-trip test over the spec examples catches this class of gap.

Acceptance Criteria

• A test loads every policy example under the PhiSQL spec/ tree and asserts no field is lost on re-serialization.
• The non-policy operation examples (SCAN, FIND_PII, SELECT_FINDINGS) are excluded, since they target Phinder.
• The test fails on each gap currently open against the schema, and is enabled as those are closed.

[jzonthemtn]

Closes a gap against the PhiSQL specification. No RFC: this is the test that would have caught every other gap in this set, and is the local equivalent of the conformance suite in philterd/phisql compliance/.