Add game-extracted UI icons: target markers, RP letters, shell set, nest, scout plane

Pulled from the game's own GameAssets/Assets/Texture2D (gitignored as a
whole, not redistributed wholesale) into a tracked assets/icons/ so
they're actually usable in FeNigma's UI instead of us redrawing
equivalents:

- targets/enemy/, targets/friendly/: one marker per game unit type
  ('Enemy_*.png' / 'Friendly_*.png', the source files spell the latter
  'Frendly_*.png', corrected on copy).
- reference_points/: the four lettered RP markers ('Refrence_Point_*.png'
  in the source, typo corrected the same way).
- shells/: one icon per Shell enum member (shells.py), named to match
  exactly so they're a direct Shell.name lookup, no mapping table
  needed. A few source files don't carry the shell's short code in
  their name (Mustard Gas_DemoBlocked -> MSTD, Tear Gas -> TEAR,
  Propaganda -> PRPG, Phosgene -> PHGN, Nuclear -> ATMC), documented in
  assets/icons/README.md.
- nest/IronNest.png: the Iron Nest/FDC unit icon.
- misc/ScoutPlane.png: fits the scout-flight planning feature.

Also commits the (already-made, previously uncommitted) .gitignore
entry for GameAssets itself, which is what makes pulling individual
icons out into a tracked directory sensible instead of contradictory.

.gitignore

@@ -2,3 +2,4 @@ __pycache__/
 *.pyc
 captures/*.png
 .venv/
+GameAssets

assets/icons/README.md

@@ -0,0 +1,25 @@
+# Icons
+
+Extracted from the game's own `GameAssets/Assets/Texture2D` (gitignored,
+not redistributed as a whole), for reuse in FeNigma's UI rather than
+redrawing equivalents from scratch. Two source filenames had typos in
+the game files themselves ("Frendly_", "Refrence_Point_"), corrected
+here on copy; everything else keeps its original name.
+
+- `targets/enemy/`, `targets/friendly/` - one marker icon per game unit
+  type, `Enemy_*.png` / `Friendly_*.png` (originally `Frendly_*.png`).
+  Not a strict 1:1 with `TargetType` in `models.py`, the game has more
+  unit types than this app currently tracks as targets, pick whichever
+  fits closest per type.
+- `reference_points/` - `ReferencePoint_A.png` .. `_D.png` (originally
+  `Refrence_Point_*.png`), the four lettered RP markers.
+- `shells/` - one icon per `Shell` enum member in `shells.py`, named to
+  match exactly (`AP.png`, `HE.png`, ...), safe to look up by
+  `Shell.name` directly. Source filenames for the ones without a game
+  short-code baked in: MSTD <- "Mustard Gas_DemoBlocked.png", TEAR <-
+  "Tear Gas.png", PRPG <- "Propaganda.png", PHGN <- "Phosgene.png",
+  ATMC <- "Nuclear.png".
+- `nest/IronNest.png` - the Iron Nest / FDC unit icon (originally
+  "IronNest Unit Icon.png").
+- `misc/ScoutPlane.png` - scout-flight icon (originally "Scout Plane
+  B.png"), fits the app's scout flight planning feature.