D252: document the rainbow particle-palette bug (no fix for v0.2.0-pre)

User playtest report: bullet-impact sparks and lingering smoke/explosion
residue drift through the full hue wheel over time instead of holding their
intended grey/orange palette. Logged in findings §F (D252) with this
session's mechanism work: particle colour comes entirely from the 15
distinct IMAGE_SMOKE_N x IMAGE_FIRE_N (RGBA16) pairs the particles age
through (assets/oddtextures.c records, white default vertex); the M-82/D172
tile-offset fix and the 5551 decode are both verified correct for this path;
prime suspect is import_texture_rgba16 deriving width/height from the tile
line size (32x7 instead of 16x14 for the 16px-wide fire tiles), which would
scramble each aging stage into a different wrong hue -- exactly the observed
slow rainbow sweep. Per user instruction: documented only, ships as a known
issue (README + release notes + GRAPHICS-BACKLOG); index csv regenerated.

.github/release-notes.md

@@ -33,6 +33,9 @@
   The most visible gap in this release.
 - **Music quality on some tracks** — a wrong-sounding bass instrument on a few
   elevator/level tracks (D230).
+- **Particle colours cycle through a rainbow palette** — bullet-impact sparks
+  and lingering smoke/explosion residue drift through the hues over time
+  instead of holding their intended grey/orange palette (D252).
 - Water on `IsWater` levels shows a moving seam between two patterns (D245);
   thin pixel strips at the left/right screen edges at non-integer window
   scales (D246); character face textures can wrap on Silo (D197).