refactor(req): clarify value comparison contract

[tisonkun]

Summary

• rename ReqValue::total_cmp to ReqValue::compare and keep the trait contract implementation-neutral
• implement primitive ReqValue types explicitly so each ordering and wire-codec mapping is visible without a macro
• compare f32 and f64 numerically with partial_cmp(...).unwrap(), reject NaN split points, and preserve numeric equality for signed zeros

Rationale

REQ already skips NaN updates and rejects NaN query and deserialized values, so its internal comparator does not need to define an ordering for NaN. For accepted floating-point values, partial_cmp(...).unwrap() matches the Java and C++ numeric comparison behavior, including treating signed zeros as equal, and fails immediately if the NaN invariant is violated.

The six built-in primitive implementations are small and stable enough to keep explicit. They reuse the existing codec error adapter but do not introduce another helper abstraction. The earlier common::float extraction and canonical-hash refactor have been removed; canonical floating-point hashing remains unchanged.

No changelog entry is added because REQ is still unreleased.

Testing

• cargo x check
• cargo x test
• cargo x lint