Backward chaining does not resolve fact-to-fact comparisons

[ops-ping]

Backward chaining silently fails any rule condition that compares two facts, while forward chaining handles the same condition correctly.

Evidence

Through vrules_core::prove (which delegates to rust_rule_engine::backward::{BackwardEngine, GRLQueryParser, GRLQueryExecutor}):

knowledge base	result
when Order.Amount <= Account.Balance	not provable, 0 proof steps
when Order.Amount <= 100.0	provable, 1 proof step

Facts {"Order.Amount": 50, "Account.Balance": 100, "Account.AgeDays": 45} in both cases.

Forward chaining, same shape, is correct in all four quadrants (Order.amount > Order.limit):

amount	limit	engine	truth
100	50	true	true
50	100	false	false
1	999	false	false
999	1	true	true

Cause

src/backward/rule_executor.rs delegates to engine::condition_evaluator::ConditionEvaluator, whose evaluate_condition compares the resolved left field against condition.value verbatim:

Ok(condition.operator.evaluate(&value, &condition.value))

The right-hand operand is never resolved against facts, so Account.Balance is compared as the literal string "Account.Balance". The forward engine has its own path in src/engine/engine.rs including resolve_value_facts(val, facts), which is exactly the resolution the backward path lacks.

This is an asymmetry in upstream rust-rule-engine, not something vrules removed: condition_evaluator.rs and the whole backward/ tree are byte-identical to rust-rule-engine.pre-purge-backup-20260802-130744.

Blast radius

ConditionEvaluator is referenced only from src/backward/ (rule_executor.rs, search.rs), so a fix is contained to backward chaining.

Impact

The Proof browser example ships a knowledge base whose first rule uses Order.Amount <= Account.Balance, so it renders NOT PROVABLE with an empty proof tree, and has been doing so on the published site.

Coverage gap

crates/vrules-core/tests/conformance_backward.rs (43 tests) contains no fact-to-fact comparison at all, which is why this was never caught.

Suggested fix

Resolve condition.value against facts before comparing, mirroring resolve_value_facts, and add backward conformance coverage for fact-to-fact in all four comparison quadrants.