SendProxy for ModSharp (CS2) — scaffold + RE writeup + public API

Port of SourceMod SendProxy/SendProxyManager to ModSharp/CS2.

- SendProxy.Shared: public ISendProxyManager API mirroring sendproxy.inc + a per-client
  (IGameClient) recipient arg for the matrix dimension.
- SendProxy core: module lifecycle, hook registry, gamedata registration, and the
  per-field encoder-swap mechanism coded against the reverse-engineered offsets
  (CNetworkSerializerFieldInfo+0x38 vtable slot 0 = the Source1 m_pProxyFn analog).
  The live memory patch is gated off (EncoderHook.Enabled=false) until the offsets are
  verified on a target build — registration-only until then, no crash risk.
- gamedata file documents the RE'd encode-path targets (EncodeField / SendClientMessages
  / WriteDeltaEntity_Internal) with string-ref anchors; linux sigs marked for verification.
- README: full CS2 reverse-engineering writeup (flattened serializer, shared-snapshot
  model, hook point, per-client feasibility) + phase plan + usage.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>