test: refine replacement property strategy

[rlippmann]

What changed

• refine the Hypothesis replacement-case strategy in tests/test_properties.py so generated deterministic replacement scenarios directly produce the authoritative initial state under test
• simplify the replacement property to compare the replacement directive against equivalent explicit public operations on that same generated initial state
• preserve coverage for old source present/absent, unrelated premise and policies, and normalization-sensitive item identities without mutating the generated state inside the test body

Why

• the prior property rewrote the generated payload before asserting, which made the invariant harder to reason about
• generating the actual state under test makes the property closer to the intended contract: replacement should match the equivalent explicit transition sequence from the same authoritative starting state

Checklist

• pre-commit run (uv run pre-commit run --all-files)
• tests pass (uv run pytest)

[rlippmann]

Closing this stacked PR because it could not be rebased cleanly after PR #204 merged. Reopened the same follow-up change from a fresh branch based on current main.