Review fixes: a failed environment source lights nothing (#2436)

Two review points on the merge:

- env_pixels returned None both for an unconfigured light (no Source, no
  Environment Layer) and for a failed source (a deleted layer, missing
  footage, a failed decode), so a selected source that failed silently
  swapped in the default studio environment — relighting and reshadowing
  the scene behind the user's back. It now tells the three apart
  (EnvPixels::Absent/Image/Failed): only the unconfigured light uses the
  default; a failed source contributes nothing, exactly like a layer whose
  source fails renders nothing. The repair the user can act on stays where
  it already lives — missing footage is flagged on the item, with the
  relink commands to fix it.

- The docs claimed only a uniform image casts no shadow. A non-uniform one
  can be directionless too: opposing bright regions cancel, and anything
  under 1% directional is rejected. agents.md, architecture.md and the
  sun/dominant_direction doc comments now say an image without a
  sufficiently strong dominant direction.

Tests: an unreadable Environment Layer (auto-picked), an unreadable
selected Source and a deleted one all light nothing — pixel-equal to the
unlit render, no studio default, no default-environment shadow — while an
unconfigured light still lights; two equal bright spots at opposite
azimuths cancel to no sun and no key, and tilting the balance re-emerges
pointing at the brighter spot.

docs/architecture.md

@@ -163,10 +163,13 @@ camera, dark below and behind, at a dim 0.3 mean level — of the light's colour
 when there is neither) is an equirectangular image (cosine irradiance map; roughness-indexed
 mip chain with the split-sum environment BRDF fit), and percentage-closer-filtered shadow maps
 (spot: perspective, parallel: orthographic over the scene, point: six cube faces, environment:
-one orthographic from the image's dominant bright region — none only when the image is
-directionless, as a uniform one is — occluding only the image's directional key fraction of
+one orthographic from the image's dominant bright region — none only when the image lacks a
+sufficiently strong dominant direction (a uniform one, or opposing bright regions that
+cancel) — occluding only the image's directional key fraction of
 the light, so the shadow stays partial like After Effects' area-light shadows, with a soft
-default penumbra of at least eight pixels or four map texels). An Environment light only lights Advanced 3D comps,
+default penumbra of at least eight pixels or four map texels). A Source or Environment Layer
+that fails to render — a deleted layer, missing footage — lights nothing: no silent fallback
+to the default studio environment. An Environment light only lights Advanced 3D comps,
 so adding one (or changing a light's type to it) switches a Classic 3D comp to Advanced 3D,
 as model layers do. Cards keep
 Material Options (Accepts Lights/Shadows, Casts Shadows On/Only, Ambient, Diffuse, Specular);