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/game/pxl8_gui.c

@@ -0,0 +1,133 @@
+#include "pxl8_gui.h"
+
+#include <stdlib.h>
+#include <string.h>
+
+#include "pxl8_gfx.h"
+
+pxl8_gui_state* pxl8_gui_state_create(void) {
+    pxl8_gui_state* state = (pxl8_gui_state*)malloc(sizeof(pxl8_gui_state));
+    if (!state) return NULL;
+
+    memset(state, 0, sizeof(pxl8_gui_state));
+    return state;
+}
+
+void pxl8_gui_state_destroy(pxl8_gui_state* state) {
+    if (!state) return;
+    free(state);
+}
+
+void pxl8_gui_begin_frame(pxl8_gui_state* state) {
+    if (!state) return;
+    state->hot_id = 0;
+}
+
+void pxl8_gui_end_frame(pxl8_gui_state* state) {
+    if (!state) return;
+
+    if (!state->cursor_down) {
+        state->active_id = 0;
+    }
+    state->cursor_clicked = false;
+}
+
+void pxl8_gui_cursor_move(pxl8_gui_state* state, i32 x, i32 y) {
+    if (!state) return;
+    state->cursor_x = x;
+    state->cursor_y = y;
+}
+
+void pxl8_gui_cursor_down(pxl8_gui_state* state) {
+    if (!state) return;
+    state->cursor_down = true;
+}
+
+void pxl8_gui_cursor_up(pxl8_gui_state* state) {
+    if (!state) return;
+    state->cursor_down = false;
+    state->cursor_clicked = true;
+}
+
+static bool is_cursor_over(const pxl8_gui_state* state, i32 x, i32 y, i32 w, i32 h) {
+    return state->cursor_x >= x && state->cursor_x < (x + w) &&
+           state->cursor_y >= y && state->cursor_y < (y + h);
+}
+
+bool pxl8_gui_button(pxl8_gui_state* state, pxl8_gfx* gfx, u32 id, i32 x, i32 y, i32 w, i32 h, const char* label) {
+    if (!state || !gfx || !label) return false;
+
+    bool cursor_over = is_cursor_over(state, x, y, w, h);
+    bool is_hot = (state->hot_id == id);
+    bool is_active = (state->active_id == id);
+
+    if (cursor_over) {
+        state->hot_id = id;
+    }
+
+    if (cursor_over && state->cursor_down && state->active_id == 0) {
+        state->active_id = id;
+    }
+
+    bool clicked = is_active && state->cursor_clicked && cursor_over;
+    if (clicked) {
+        state->active_id = 0;
+    }
+
+    u8 bg_color;
+    u8 border_color;
+    i32 offset_x = 0;
+    i32 offset_y = 0;
+
+    if (is_active) {
+        bg_color = 4;
+        border_color = 3;
+        offset_x = 1;
+        offset_y = 1;
+    } else if (is_hot || cursor_over) {
+        bg_color = 4;
+        border_color = 8;
+    } else {
+        bg_color = 3;
+        border_color = 4;
+    }
+
+    pxl8_2d_rect_fill(gfx, x, y, w, h, bg_color);
+    pxl8_2d_rect(gfx, x, y, w, h, border_color);
+
+    i32 text_len = (i32)strlen(label);
+    i32 text_x = x + (w / 2) - ((text_len * 8) / 2) + offset_x;
+    i32 text_y = y + (h / 2) - 5 + offset_y;
+    pxl8_2d_text(gfx, label, text_x, text_y, 6);
+
+    return clicked;
+}
+
+void pxl8_gui_window(pxl8_gfx* gfx, i32 x, i32 y, i32 w, i32 h, const char* title) {
+    if (!gfx || !title) return;
+
+    pxl8_2d_rect_fill(gfx, x, y, w, 28, 1);
+    pxl8_2d_rect_fill(gfx, x, y + 28, w, h - 28, 2);
+    pxl8_2d_rect(gfx, x, y, w, h, 4);
+    pxl8_2d_rect_fill(gfx, x, y + 28, w, 1, 4);
+
+    i32 title_x = x + 10;
+    i32 title_y = y + (28 / 2) - 5;
+    pxl8_2d_text(gfx, title, title_x, title_y, 8);
+}
+
+void pxl8_gui_label(pxl8_gfx* gfx, i32 x, i32 y, const char* text, u8 color) {
+    if (!gfx || !text) return;
+    pxl8_2d_text(gfx, text, x, y, color);
+}
+
+bool pxl8_gui_is_hovering(const pxl8_gui_state* state) {
+    if (!state) return false;
+    return state->hot_id != 0;
+}
+
+void pxl8_gui_get_cursor_pos(const pxl8_gui_state* state, i32* x, i32* y) {
+    if (!state) return;
+    if (x) *x = state->cursor_x;
+    if (y) *y = state->cursor_y;
+}