Prefix plugin logs with LocalPlayerId

Add a per-instance local-player log prefix and apply it to networking and main plugin logs to make shared Fallout4Together.log entries readable when multiple instances run. Introduces F4T::Networking::GetLocalPlayerLogPrefix() (declared in F4TNetworking.h, implemented in F4TNetworking.cpp) and updates numerous REX::INFO/WARN calls to include the prefix. Also: only log sent player transforms after SendTransformPacket succeeds, add a small TODO comment about per-instance log files, and update docs (dev-log.md and plugin/setup.md) and the setup checklist to reflect the prefixed logging behavior. The prefix reads the assigned player ID via the thread-safe remote-player accessor and defaults to "[LocalPlayerId=unassigned]" before assignment.

docs/dev-log.md

@@ -516,6 +516,34 @@ Player position: X=2048.00, Y=2048.00, Z=0.00, AngleZ=0.00
 
 ---
 
+## 2026-05-31 - Per-Instance Client Log Prefixes
+
+### What Changed
+
+- Added a shared local-player log prefix helper for plugin networking logs.
+- Prefixed important startup, connection, welcome, transform send, remote update, disconnect, and networking warning logs with `[LocalPlayerId=unassigned]` or `[LocalPlayerId=N]`.
+- Kept the existing `Fallout4Together.log` file name and avoided per-instance log files for this milestone.
+
+### What Worked
+
+- The prefix reads the existing assigned player ID state through the thread-safe remote-player state accessor.
+- Remote player state storage and packet relay behavior remain unchanged.
+
+### What Broke
+
+- Nothing recorded.
+
+### Notes
+
+- Actor spawning, actor movement, gameplay sync, and protocol changes remain out of scope.
+- A future launcher/profile milestone can revisit separate per-instance log files.
+
+### Next Steps
+
+- Test two Fallout 4 instances through F4SE and confirm prefixed remote update/disconnect logs are readable in the shared log file.
+
+---
+
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