Bound batch decode by item limit (PLT-820) - #98

The fix is sound: parseMessage now stops one element past the item limit, which correctly preserves handleBatch's strict len(msgs) > limit rejection while bounding per-element allocations, and it's wired into every built-in codec path (WS, IPC/stdio via ServeCodec, and HTTP via the new attachHandler in serveSingleRequest). No blocking issues found; a few non-blocking notes on latent budget coupling on the HTTP path, allocation-test robustness, and the unexported setHandler extension point.