test: widen property tests beyond in-memory, one-fact-per-transact queries #386

Problem

tests/property_test.rs runs 8M cases a night, but it covers a small space:

• It has 3 properties in 218 lines.
• It uses Minigraf::in_memory() only: no file, checkpoint or reopen.
• It writes one fact per transact, which avoids bug: same-transaction multi-valued facts read back as ONE value, differing between EAVT and AEVT; a batched retract can leave retracted values visible #371 by design.
• It has no retracts, no :as-of, no :valid-at, and no joins beyond one clause plus not.
• A query that returns Err is counted as an empty result (if result.is_err() { return vec[]; }), so errors can hide mismatches.

Ask

• Treat an unexpected query error as a test failure.
• Add a file-backed variant that checkpoints and reopens before it queries.
• Batch facts into random-sized transactions.
• Add retracts, and temporal queries checked against the reference evaluator.
• Add a join property (two or three clauses) checked against the reference evaluator.
• Lower PROPTEST_CASES if needed. A wider space with fewer cases finds more bugs than 8M cases in a small space.

Related: the model-based test in the sibling issue covers operation sequences. This issue covers query semantics.