[draft] feat(monitoring): add per-channel and per-client byte metrics

[gimballock]

Add bytes_received/bytes_sent tracking across all three components (pool, jd-client, translator) for SV2 channels and SV1 clients.

SV2 byte tracking:

• Per-channel (bytes_received, bytes_sent) counters in pool, jd-client, and translator, keyed by channel_id in bytes_by_channel maps
• Shared mining_message_channel_id() helper in protocol_message_type.rs to extract channel_id from parsed Mining messages
• Raw frame payload guard (len >= 4) for channel_id extraction from unparsed upstream frames in jd-client and translator
• Cleanup in all CloseChannel handlers and ChannelManagerData::reset()
• JDC record_upstream_sent_bytes() eliminates double-lock on sends

SV1 byte tracking (translator only):

• Arc counters in ConnectionSV1, shared with DownstreamData
• Reader counts line.len()+1 (newline stripped by LinesCodec)
• Writer counts serialized JSON bytes before write_all

Prometheus metrics:

• sv2_server_channel_bytes_{received,sent}_total
• sv2_client_channel_bytes_{received,sent}_total
• sv1_client_bytes_{received,sent}_total
• All use gauge-with-reset pattern for stale label cleanup

Also updates README with new metric names and design notes on capacity planning, anomaly detection, cost attribution, and reflection attack detection use cases.