M3: Asset pipeline — ContentPaths + ContentLoader + asset-test

Formalises Content/ access from the Godot host. Content lives at the
repo root (sibling of Theriapolis.Godot/), not duplicated under res://,
so the MonoGame branch and headless Tools keep reading from the same
single source of truth.

ContentPaths.cs:
  Static ContentRoot/DataDir/GfxDir resolved once via res:// walk-up
  and cached. Replaces two inline ResolveDataDir copies in SmokeTest
  and WorldMapView.

ContentLoader.cs:
  LoadGfx(relativePath) -> ImageTexture, cached by relative path.
  Bypasses the res:// import pipeline because Content/ lives outside
  the project — fine for static pixel-art assets at native size, and
  the project default texture filter is already Nearest. Cache is
  per-process, never evicted (full atlas <1 MB).

AssetTest.cs + Main.cs --asset-test flag:
  Smoke-tests the pipeline. Walks Content/Data and Content/Gfx,
  reports counts, attempts to load every PNG, prints per-subdir
  breakdown. Quits with non-zero on any failure.

Verified post-refactor (--asset-test):
  53 JSON files in Data, 50 PNG files in Gfx (8 tactical/deco +
  42 tactical/surface), 50/50 loaded, 0 failures.

Verified no regressions:
  --smoke-test (M1) still produces canonical FNV hashes.
  --world-map 12345 (M2) still produces 3 rivers / 91 roads /
  226 settlements / 0 rails / 0 bridges.

Scope note: plan mentioned "tile/NPC/CodexUI atlases — three
separate themes". Only tactical/ exists in Content/Gfx today; NPC
and CodexUI atlases never landed during MonoGame development. M3
ships what's actually present. ContentLoader.LoadGfx works for any
future sub-directories without changes.

Closes M3 of theriapolis-rpg-implementation-plan-godot-port.md.
Next: M4 (tactical render).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Theriapolis.Godot/Main.cs

@@ -1,4 +1,5 @@
 using Godot;
+using Theriapolis.GodotHost.Platform;
 using Theriapolis.GodotHost.Rendering;
 
 namespace Theriapolis.GodotHost;
@@ -18,6 +19,7 @@ public partial class Main : Node
 
         ulong? smokeTestSeed = null;
         ulong? worldMapSeed = null;
+        bool runAssetTest = false;
         for (int i = 0; i < args.Length; i++)
         {
             if (args[i] == "--smoke-test")
@@ -28,6 +30,11 @@ public partial class Main : Node
                 smokeTestSeed = seed;
                 break;
             }
+            if (args[i] == "--asset-test")
+            {
+                runAssetTest = true;
+                break;
+            }
             if (args[i] == "--world-map")
             {
                 ulong seed = 12345UL;
@@ -45,6 +52,13 @@ public partial class Main : Node
             return;
         }
 
+        if (runAssetTest)
+        {
+            int code = AssetTest.Run();
+            GetTree().Quit(code);
+            return;
+        }
+
         if (worldMapSeed.HasValue)
         {
             // Replace the M0 hello-world children with the M2 world-map view.