Refactor server into reusable core and GUI

Split the monolithic test server into a reusable server core and a thin terminal launcher, and add a PySide6 developer GUI. Added server_core.py (FalloutTogetherServer) with lifecycle control, thread-safe client snapshots, stats and log listener support; added client_session.py for per-client state; added dev_server_app.py GUI and requirements.txt. Updated server.py to use the new server core, and revised server/README.md and docs/dev-log.md to document the new structure, usage, and validation steps. Protocol behavior (welcome/transform/disconnect handling and newline-separated JSON) remains unchanged.

docs/dev-log.md

@@ -882,6 +882,102 @@ Player position: X=2048.00, Y=2048.00, Z=0.00, AngleZ=0.00
 
 ---
 
+## 2026-06-02 - Reusable Server Core Refactor
+
+### What Changed
+
+- Split the Python relay server into a reusable server core and a thin terminal
+  launcher.
+- Added `server/client_session.py` to track connected client state, packet
+  counters, last packet time, and last transform snapshots.
+- Added `server/server_core.py` with `FalloutTogetherServer` lifecycle methods,
+  log listeners, client snapshots, server stats, and thread-safe shared state.
+- Updated `server/README.md` with the new server structure and validation
+  commands.
+
+### What Worked
+
+- The terminal command remains `cd server` followed by `python server.py`.
+- The server still defaults to `127.0.0.1:7777`.
+- Newline-separated JSON handling, welcome packets, transform mutation,
+  transform relay, disconnect relay, and unknown transform field preservation
+  remain protocol-compatible.
+- The refactor prepares the server for a future desktop dev-server UI without
+  adding PySide6, fake-player management, movement scripts, or gameplay
+  features.
+
+### What Broke
+
+- Nothing recorded during implementation.
+
+### Notes
+
+- The future UI can use `FalloutTogetherServer.start()`, `stop()`,
+  `is_running()`, `get_clients()`, `get_stats()`, and log listener callbacks
+  without scraping terminal output.
+- Fallout 4 plugin code and network protocol fields were left unchanged.
+
+### Next Steps
+
+- Run compile checks for the server files.
+- Validate server startup, fake-client welcome handling, transform relay, and
+  disconnect relay with fake clients and Fallout 4 plugin clients.
+
+---
+
+## 2026-06-02 - Dev Server GUI
+
+### What Changed
+
+- Added a PySide6 desktop developer GUI for the reusable Python server core.
+- Added a Server Console tab with start/stop controls, live server logs, live
+  stats, and a connected-client table.
+- Added a Fake Clients tab placeholder for future fake-client management and
+  movement-script controls.
+- Added a server-local requirements file for the GUI dependency.
+- Updated server documentation with GUI install and run instructions while
+  preserving the terminal server flow.
+
+### What Worked
+
+- The GUI uses `FalloutTogetherServer` instead of duplicating networking logic.
+- Server logs flow through `add_log_listener(...)` and a Qt signal bridge rather
+  than stdout scraping.
+- Stats and connected-client snapshots are refreshed through `get_stats()` and
+  `get_clients()` on a `QTimer`.
+- The terminal launcher, fake client, network protocol, and plugin code were
+  left unchanged.
+
+### What Broke
+
+- Nothing recorded during implementation.
+
+### Notes
+
+- Fake client spawning and movement scripts remain intentionally unimplemented.
+- Full GUI interaction and Fallout 4/F4SE validation still need to be run
+  manually on the desktop.
+
+### Testing
+
+- Ran `python -m py_compile server.py server_core.py client_session.py fake_client.py dev_server_app.py`
+  from `server`; it succeeded.
+- Checked IDE lints for the new GUI and edited docs; no diagnostics were
+  reported.
+- Checked the current Python environment for `PySide6`; it is not installed, so
+  `pip install -r requirements.txt` is required before launching the GUI.
+- Did not start a duplicate terminal server because an existing `python
+  server.py` session and `python fake_client.py` session were already present.
+
+### Next Steps
+
+- Run `python dev_server_app.py`, click `Start Server`, and connect
+  `fake_client.py`.
+- Launch Fallout 4 through F4SE and confirm the plugin appears in the GUI client
+  table with transform stats updating.
+
+---
+
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