test: add proptest coverage for EntityEvents and idempotency_guard

[nicolasburtey]

Summary

Adds two property-based test files covering the framework's core contracts.

tests/proptest_events.rs — invariants on EntityEvents:

• mark_persisted_sequencing — across arbitrary push/extend/mark interleavings, persisted events have contiguous monotonic sequences from 1 and the new-events buffer is drained.
• load_first_roundtrip — events serialized through GenericEvent and reloaded compare equal in order.
• load_n_grouping — K entities × M events each: returned count = min(K, n), more flag matches, no event lost or stitched across ids.
• iter_all_ordering — invariant after every step: iter_all yields exactly the cumulative push order regardless of when marks occur.

tests/proptest_idempotency.rs — the idempotency_guard! macro contract:

• stutter_is_idempotent — c; c ≡ c for any command, both already_applied-only and resets_on forms.
• full_retry_is_noop_for_pure_commands — replaying any sequence of already_applied-only commands pushes zero new events. The retry-safety property callers depend on.
• pure_commands_dedup_by_first_occurrence — extensional pin: stream equals deduplicated first-occurrence subsequence of pushed events.
• resets_on_allows_alternation — Activate/Deactivate alternations never produce two consecutive identical states (proves resets_on actually resets the scan window).

Each property runs 4096 cases. Total runtime ~0.22s in release. No production code touched.

Test plan

• cargo test --test proptest_events --test proptest_idempotency --release passes locally
• CI green