Add HTLC forward counters for client channel monitoring

[amackillop]

Count success/failure for HTLC forwards that touch private (client) channels. Public-to-public forwards are ignored.

Counters are plain AtomicU64s so ldk-node doesn't pull in prometheus. The app layer reads snapshots via Node::forward_counters() and maps them to whatever it wants.

Each forward is classified by direction (to_client vs from_client) and outcome (success, downstream failure, local failure). A separate counter tracks UnknownNextHop/InvalidForward hits, which point at SCID alias tracking bugs.

Channel lookup uses is_announced on both sides. If a channel is gone by the time the event fires, we assume private. Safe default for an LSP.

[martinsaposnic]

reviewed with the help of claude,

 ┌──────────┬───────┬─────────────────────────────────────────────────────────────────────────────────────────┐
  │ Severity │ Count │                                          Items                                          │
  ├──────────┼───────┼─────────────────────────────────────────────────────────────────────────────────────────┤
  │ Major    │ 1     │ Clone on HTLCHandlingFailureType needs verification                                     │
  ├──────────┼───────┼─────────────────────────────────────────────────────────────────────────────────────────┤
  │ Minor    │ 3     │ channels scope widened, unknown-channel default silent, ForwardSnapshot not re-exported │
  ├──────────┼───────┼─────────────────────────────────────────────────────────────────────────────────────────┤
  │ Nit      │ 2     │ Relaxed ordering (fine), boolean trap in record_failure                                 │
  └──────────┴───────┴─────────────────────────────────────────────────────────────────────────────────────────┘

about the "major" item, did cargo check locally and it works. minor issues and nits are safe to ignore