World: replace skybox changer with a PNG skybox image

sv_skyname did not take effect at runtime in this build, so the Skybox card now
replaces the skybox materials' textures with <config dir>/skybox.png.

The PNG is decoded through libpng16's simplified API (dlopen'ed, so no new link
dependency) and uploaded with IMaterialSystem::CreateTextureFromBits as an
RGBA8888 texture. IMaterialVar::SetTextureValue assigns it to $basetexture and
the HDR variants for every material in the SkyBox textures group (all six
faces get the same image).

Original textures are recorded per material/var (texture pointer plus string
fallback) and restored when the feature is off. Records survive level changes
so a reused sky material is still restorable; the override is re-applied to a
new map automatically. The card shows the image path and has a Reload image
button; a missing/unreadable PNG raises an error toast.

1 files changed, 19 insertions, 14 deletions

diff --git a/CONTEXT.md b/CONTEXT.md
index f2b091a..a490403 100644
--- a/CONTEXT.md
+++ b/CONTEXT.md
@@ -358,20 +358,25 @@ vocabulary / decisions.
 - End is clamped above Start. Re-applied on level change via
   `fog_invalidate()`. Saved as `[fog] active/bind/r/g/b/start/end/skybox/disable`.
 
-### Skybox changer
-
-- `sv_skyname <name>` through `ClientCmd`, the same mechanism the stock
-  `skybox_swapper` entity uses. Choice list is the HL2 skybox set from the
-  game's VPK (no episode-only skies, so a selected sky always exists).
-- Before the first override of a map, the map's own sky is captured by
-  scanning loaded materials in the `SkyBox textures` group and parsing
-  `skybox/<name>(_hdr|_ldr)?(rt|bk|lf|ft|up|dn)`. Disabling the feature (or
-  choosing "Map default") restores that name; a level change also restores it
-  naturally, so `skybox_invalidate()` just drops the captured state.
-- Changing the sky invalidates the world tint so the freshly loaded sky
-  materials get the sky colour.
-- Saved as `[skybox] active/bind/choice`. The GUI uses a `CK_CHOICE`
-  control (value box with `<`/`>` hit zones).
+### Skybox image
+
+- Replaces the map skybox with `<config dir>/skybox.png` (same directory as
+  the INI). Not `sv_skyname`: that did not take effect at runtime in this
+  build, so the skybox card now works on materials instead.
+- PNG is decoded with libpng16's simplified API (`dlopen`ed, no link
+  dependency); the RGBA pixels become an `ITexture` via
+  `IMaterialSystem::CreateTextureFromBits` (RGBA8888, one mip).
+- The texture is assigned with `IMaterialVar::SetTextureValue` to
+  `$basetexture`, plus `$hdrbasetexture` / `$hdrcompressedtexture` when those
+  vars exist, on every material in the `SkyBox textures` group or named
+  `skybox/*`. The same image is mapped onto all six faces.
+- Original textures are recorded per material/var (texture pointer plus the
+  string as a fallback) and restored when the feature is disabled. Records
+  survive level changes so a reused sky material can still be restored; the
+  override is re-applied to the new map automatically.
+- The card shows the image path and has a Reload image button. A missing or
+  unreadable PNG raises an error toast; the image is only re-read on Reload.
+- Saved as `[skybox] active/bind`.
 
 ## Logging