fix(worker): hash-partition by ordering_key for multi-worker fifo

[zeybek]

Summary

Fix multi-worker partitioning so that messages sharing an ordering_key route to a single worker. Switch from id % total_workers to abs(hashtext(ordering_key)) % total_workers when the key is set; id-based fallback preserves balance for unkeyed messages.

Why

In src/worker/batch_processor.c:460, id-based modulo spread siblings of the same ordering_key across worker partitions. Per-key FIFO was only enforced by a NOT EXISTS subquery, which produced starvation and unnecessary cross-worker coordination under N>1 workers. Hash-partitioning the key restores per-key FIFO at the partition level.

Validation

• make installcheck-regress — 27/27 regress + 13/13 isolation (PG18, all protocols)
• make installcheck-tap — 3/3 (worker lifecycle intact)
• New isolation spec ordering_key_partition passes — with hash('partition-X') % 2 = 0, w0 locks 5 rows, w1 locks 0
• Spec registered in Makefile ISOLATION list
• make format / make lint / make tidy (verified by CI)

Checklist

• Isolation test added (tests/isolation/specs/ordering_key_partition.spec)
• Migration file added (sql/ulak--0.0.2--0.0.3.sql — marker, no schema change)
• Unkeyed messages keep id-based fallback for load balance
• Not a user-facing behavior change; no docs update required
• Commit message follows Conventional Commits