refactor: reorganize pxl8 into client/src/ module structure

- core/: main entry, types, logging, I/O, RNG
- asset/: ase loader, cart, save, embed
- gfx/: graphics, animation, atlas, fonts, tilemap, transitions
- sfx/: audio
- script/: lua/fennel runtime, REPL
- hal/: platform abstraction (SDL3)
- world/: BSP, world, procedural gen
- math/: math utilities
- game/: GUI, replay
- lua/: Lua API modules

client/src/gfx/pxl8_blit.c

@@ -0,0 +1,57 @@
+#include "pxl8_blit.h"
+
+void pxl8_blit_hicolor(u16* fb, u32 fb_width, const u16* sprite, u32 atlas_width,
+                       i32 x, i32 y, u32 w, u32 h) {
+    u16* dest_base = fb + y * fb_width + x;
+    const u16* src_base = sprite;
+
+    for (u32 row = 0; row < h; row++) {
+        u16* dest_row = dest_base + row * fb_width;
+        const u16* src_row = src_base + row * atlas_width;
+
+        u32 col = 0;
+        u32 count2 = w / 2;
+        for (u32 i = 0; i < count2; i++) {
+            u32 pixels = ((const u32*)src_row)[i];
+            if (pixels == 0) {
+                col += 2;
+                continue;
+            }
+            dest_row[col]     = pxl8_blend_hicolor((u16)(pixels), dest_row[col]);
+            dest_row[col + 1] = pxl8_blend_hicolor((u16)(pixels >> 16), dest_row[col + 1]);
+            col += 2;
+        }
+        if (w & 1) {
+            dest_row[col] = pxl8_blend_hicolor(src_row[col], dest_row[col]);
+        }
+    }
+}
+
+void pxl8_blit_indexed(u8* fb, u32 fb_width, const u8* sprite, u32 atlas_width,
+                       i32 x, i32 y, u32 w, u32 h) {
+    u8* dest_base = fb + y * fb_width + x;
+    const u8* src_base = sprite;
+
+    for (u32 row = 0; row < h; row++) {
+        u8* dest_row = dest_base + row * fb_width;
+        const u8* src_row = src_base + row * atlas_width;
+
+        u32 col = 0;
+        u32 count4 = w / 4;
+        for (u32 i = 0; i < count4; i++) {
+            u32 pixels = ((const u32*)src_row)[i];
+            if (pixels == 0) {
+                col += 4;
+                continue;
+            }
+            dest_row[col]     = pxl8_blend_indexed((u8)(pixels), dest_row[col]);
+            dest_row[col + 1] = pxl8_blend_indexed((u8)(pixels >> 8), dest_row[col + 1]);
+            dest_row[col + 2] = pxl8_blend_indexed((u8)(pixels >> 16), dest_row[col + 2]);
+            dest_row[col + 3] = pxl8_blend_indexed((u8)(pixels >> 24), dest_row[col + 3]);
+            col += 4;
+        }
+        for (; col < w; col++) {
+            dest_row[col] = pxl8_blend_indexed(src_row[col], dest_row[col]);
+        }
+    }
+}