perf: reduce stream allocations and preserve JSON precision

[jo-cube]

• Preserve JSON number precision by keeping NDJSON values as json.RawMessage.
• Reuse bounded Kcat key storage to reduce tombstone workload allocations.
• Return Pebble value-read errors before emitting scan records.
• Add regression tests, focused benchmarks, and documentation.

Storage format, durability, and stream ordering are unchanged.