World: add world/sky material colour modulation

Port Amalgam's modulation approach: obtain IMaterialSystem from the already
loaded materialsystem.so (VMaterialSystem080) and walk materials with
FirstMaterial/NextMaterial/GetMaterial. ColorModulate the "World textures"
group (sky names excluded) and the "SkyBox textures"/sky-named group.

The walk runs only when the enabled groups or colours change, restores white
when disabled, and re-applies after level changes. Before touching materials,
GetNumMaterials() and the first material's name/group are sanity-checked so a
wrong vtable slot logs and aborts instead of walking garbage.

GUI: "World tint" card with world/sky toggles and RGB sliders; config section
[worldtint].

1 files changed, 22 insertions, 3 deletions

diff --git a/CONTEXT.md b/CONTEXT.md
index fb97636..ce4a02e 100644
--- a/CONTEXT.md
+++ b/CONTEXT.md
@@ -295,14 +295,33 @@ vocabulary / decisions.
 
 - Visuals card with the usual Active + Bind. Unbound and Active means always
   on; a bind makes it a hold-to-brighten key.
-- Applies the client `mat_fullbright` cvar to 1/0 through `VEngineCvar004`
-  (`FindVar` slot 12, `ConVar::SetValue(int)` slot 11), which bypasses the
-  console cheat gate. No server-side writes.
+- Applies the client `mat_fullbright` cvar to 1/0 through
+  `IVEngineClient::ClientCmd` (slot 7), which bypasses the console cheat gate
+  and avoids the crashing direct `ConVar` vtable path. No server-side writes.
 - Re-applied after map loads/respawns (`fullbright_invalidate` on local entity
   change) and saved as `[fullbright] active/bind`.
 - `fullbright: on/off` is logged; a missing cvar interface raises an error
   toast instead of failing silently.
 
+### World tint (material colour modulation)
+
+- Same idea as Amalgam's world modulation: find `IMaterialSystem` in the
+  already-loaded `materialsystem.so` and walk every material with
+  `FirstMaterial` / `NextMaterial` / `GetMaterial`.
+- `IMaterial::ColorModulate(r,g,b)` is applied to materials whose
+  `GetTextureGroupName()` is `World textures` (sky names excluded) and to
+  `SkyBox textures` (or names containing `sky`). Lightmaps are not touched.
+- Slot map (2013 SDK layout, registered under `VMaterialSystem080`):
+  `IMaterialSystem` 73/74/75/76/77 = First/Next/Invalid/Get/GetNumMaterials;
+  `IMaterial` 0/1/28 = GetName/GetTextureGroupName/ColorModulate.
+- `src/world.cpp`; the walk runs only when the enabled groups or colours
+  change, and re-applies after level changes via `world_tint_invalidate()`.
+  Disabling restores white, so no reload is needed.
+- Validation before any walk: `GetNumMaterials()` must return 1..200000; the
+  first material's name/group must be printable strings. A mismatch logs and
+  toasts instead of poking garbage.
+- Saved as `[worldtint] active/bind/world/sky/wr..wb/sr..sb`.
+
 ## Logging
 
 - `/tmp/hl2hake.log` is opened once at startup with `"w"` (so it only contains