cleanup: merge-readiness pass (safe fixes) + review doc

Applied verified-safe fixes from a merge-readiness review:
- NativeUtil.IsUserPtr is now platform-aware (was Linux-only (p>>40)==0x7F → would
  mis-classify Windows pointers; Linux branch byte-identical, Windows branch added,
  OS check cached).
- Removed RecipientCapture hot-path diagnostic logging (RECIP#N + _count field).
- Removed dead SendClientMessages resolution (discarded result) + its key const.
- Surfaced ForceResend injected/skippedFull counters (logged on Uninstall).
- Removed the no-op SendProxyManager.Clear() + its Shutdown call.

Dismissed 3 reviewer over-flags with evidence (IVirtualHook not IDisposable; public
module is the convention; Float32 *(double*) is the intentional encoder convention).
Recorded recommended pre-merge polish (IsValidEntity guards, API visibility) in
docs/MERGE_READINESS.md. Builds clean.

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