[carrdylan52]

The groundwork the settings menu needs: config.json can now change while the game runs, applies at once, and is saved safely. Four commits on current port. Nothing under ps2/ changes, and there's no menu yet; the Options screen is the next PR, on top of this one.

What it is

• One entry point. ConfigChange(Config) (port/src/platform/config.hpp) takes a whole Config, typically ConfigGet() with some fields edited. It does five things:

  1. It sends the settings through the serialiser and parser and back, so a bad value is reported and becomes its default, exactly as at start-up. There is no second copy of the range checks.
  2. It returns at once if nothing changed.
  3. It makes the new settings current.
  4. It saves config.json: written to config.json.tmp, then renamed over the old file, so a crash can't leave half a file.
  5. It calls each change hook with the old and the new settings.

  A failed save is retried the next time the same settings come in. ConfigChange and the hooks run on the main thread only; a hook that calls ConfigChange is refused.

• Change hooks. ConfigAddChangeHook / ConfigRemoveChangeHook. They run over a copy of the list, so a hook may remove itself. When the game's own options move into config.json (the Options PR), they register a hook; ConfigChange doesn't change.

• What applies live (ApplyConfigChange in main.cpp, where start-up already applied these):

  • master volume;
  • the input section (bindings, mouse, stick; InputApplyConfig, split out of InputInit; physical key state survives a rebind);
  • tick rate;
  • interpolation, max_fps and show_fps;
  • present mode and vsync (gfx::SetPresentMode recreates the swapchain as a resize does);
  • window size and fullscreen (WindowSetMode, the same placement as WindowInit; a maximised window is restored first);
  • aspect and ui_scale (gfx::SetFrameLayout, held until the next pump outside a frame).

  The detail and shadow distances already read live.

• Restart only. game.debug_mode applies on the next start, and ConfigAppliesOnRestart says so. The debug-toggle chord now reads the value from start-up as well.

• --width, --height and --show-fps still win over the file, and a headless window keeps its size.

As you said: no migration and no version field. A saved tick_rate of 50 still loads as 50, and that test is unchanged. Rewriting the file drops comments, which docs/PC.md now says. A separate commit fixes docs/PC.md where it still gave the tick rate as 50 Hz.

Testing

Windows (Debug, clang 23): 102 of 102. Linux (clang 23): a clean build; ctest 95 of 95 on RADV and on lavapipe; truncations.py --check passes; Doxygen gives no new warning; the two recorded replays (a debug item grant, a dungeon floor) run to the end. The new test PlatformConfig.ChangeAppliesAndSaves runs in about 0.06 s. It covers:

• a bad value falling back to its default;
• hook order, including a hook that removes itself;
• a re-entrant change being refused;
• an unchanged call doing nothing;
• a failed save being retried, made to fail by putting a directory where config.json goes, before and after load;
• no .tmp being left behind;
• a reload giving the saved values.

Changing settings live was exercised through a temporary hook in a running game, which is not part of this PR:

• ui_scale 1.5 and back;
• fullscreen on a 3840×2160 monitor and back to 1280×720;
• a maximised window resized to 1024×768;
• a tick rate of 30, then 60 (measured at 30.00 and 59.94 ticks per second);
• fifo → mailbox → immediate → fifo.

macOS is not built here.

Things you may want different

1. Every key is written, defaults included, as before. Writing only what differs from the default would let a later change of default reach players (it's how tick_rate 50 stuck), but that is a separate policy.
2. The render scale is still fixed at start-up: neither this nor a manual resize recalculates it. That belongs with the renderer.
3. git clang-format wants <nlohmann/json.hpp> ahead of the standard headers in config.cpp. That is the existing order on port; this only adds <utility> next to them, so I left it.