DDIR halfjoin: validate uniform key ranges before fanout

[frankmcsherry]

For a hashed identifier range, batch the arranged-key endpoint check and eligible update/representative checks, then emit time-eligible matches without checking every output candidate's key. Sorted real keys make equal endpoints a certificate for the range; genuine collisions retain the bounded fallback. Singleton ranges retain the pooled cross-chunk comparison path.

Targets master-next and includes #945 as a dependency. Review this PR's own change at 4e8cd894. Validation runs within each run/arranged-chunk pair; arranged-chunk seeks advance once after all runs in the block. The range proof and fallback share indexed equality grouped by source-column pair through equal_idx, preserving request order.

Validation on mini-02, Rust 1.95: cargo test --locked --offline -p interactive -p ddir-server passes 193 tests with five existing ignores; cargo clippy --locked --offline -p interactive -p ddir-server --all-targets passes with repository warnings. The halfjoin integration test compares delta joins and recursive reachability with binary joins over 24 update/retraction ticks, with one and three workers.

Remains a draft because earlier native M4 controls showed a fanout/singleton tradeoff: pair keys with fanout 64 took 1,705 → 1,158 ns/key, while singleton-list keys regressed 49.1 → 53.7 ns/key. Triangle load was roughly neutral (78.8 → 79.6 ms). These measurements predate the per-run traversal; benchmarks were not rerun for this rebase.