starkernel: item 4.3.1 -- framebuffer orientation test, found and fixed a real color-swap bug

Adds fb_draw_orientation_test() (framebuffer.c/.h): fills the four raster
corners RED/GREEN/BLUE/YELLOW via fb_fill_rect. Wired into kernel_main.c
calling fb_init() directly -- console_fb_init()/vt100_init() removed from
the boot path, since vt100.c/console.c are superseded by the Console
drawing-fabric redesign (FABRIC.md ss27) and should not be exercised even
incidentally.

The diagnostic caught a real, pre-existing bug on its first run: framebuffer.c's
pack_pixel() had its FB_PIXEL_RGBX32/FB_PIXEL_BGRX32 branches swapped relative
to UEFI GOP's own byte-order naming convention, producing a clean R<->B channel
swap (G unaffected). Spatial placement was already correct -- no flip/rotation.
Fixed by swapping pack_pixel's two return bodies to match framebuffer.h's
already-correct doc comments; kernel_main.c's GOP-format switch needed no change.

Also item 4.3.2 -- QEMU screenshot capability. scripts/qemu_screenshot.sh
already existed (monitor socket + socat + HMP screendump), just unwired and
unused this session. Redirected its PNG output to a new top-level fb/
directory (tracked in git, not logs/, not a gitignored temp dir) and added a
python3+PIL fallback for PPM->PNG conversion since imagemagick isn't
installed here. Left as a standalone script for now, not wired into a
Makefile target.

FABRIC.md items 4.3.1 and 4.3.2 marked done with acceptance evidence.

src/starkernel/hal/framebuffer.c

@@ -66,17 +66,27 @@ static FbState g_fb;
  * Internal helpers
  * --------------------------------------------------------------------- */
 
-/* Convert our 0x00RRGGBB to the hardware BGRX / RGBX 32-bit word */
+/*
+ * Convert our 0x00RRGGBB to the hardware 32-bit word.
+ *
+ * GOP format names describe increasing byte address in memory (little-endian
+ * uint32 store), e.g. PixelRedGreenBlueReserved8BitPerColor means byte0=R,
+ * byte1=G, byte2=B, byte3=X -- which as a little-endian uint32 puts R in
+ * bits[7:0], G in bits[15:8], B in bits[23:16]. The two branches below were
+ * previously swapped relative to that (found via FABRIC.md item 4.3.1's
+ * orientation test: corners rendered with R and B channels exchanged).
+ */
 static uint32_t pack_pixel(uint32_t rgb)
 {
     uint32_t r = FB_R(rgb);
     uint32_t g = FB_G(rgb);
     uint32_t b = FB_B(rgb);
     if (g_fb.fmt == FB_PIXEL_RGBX32) {
-        return (r << 16) | (g << 8) | b;       /* RGBX: R@[23:16] */
+        /* RGBX byte order R,G,B: byte0=R@[7:0] byte1=G@[15:8] byte2=B@[23:16] */
+        return (b << 16) | (g << 8) | r;
     }
-    /* BGRX (default): B@[23:16] G@[15:8] R@[7:0] */
-    return (b << 16) | (g << 8) | r;
+    /* BGRX (default), byte order B,G,R: byte0=B@[7:0] byte1=G@[15:8] byte2=R@[23:16] */
+    return (r << 16) | (g << 8) | b;
 }
 
 /* -----------------------------------------------------------------------
@@ -197,6 +207,42 @@ void fb_draw_glyph(uint32_t px, uint32_t py, uint8_t ch,
     }
 }
 
+/* -----------------------------------------------------------------------
+ * Boot diagnostic: orientation test pattern
+ * --------------------------------------------------------------------- */
+
+/**
+ * fb_draw_orientation_test — one-time boot diagnostic (FABRIC.md item 4.3.1).
+ *
+ * Fills each raster corner with a distinct solid color so a screendump
+ * immediately reveals whether the image is flipped or rotated. Not part of
+ * the Console drawing fabric itself -- diagnostic-only, analogous to item
+ * 4.2's self-test scaffolding.
+ *
+ * Corner colors (raster/top-left-origin coordinates, pre-Cartesian-flip):
+ *   top-left     RED
+ *   top-right    GREEN
+ *   bottom-left  BLUE
+ *   bottom-right YELLOW
+ */
+void fb_draw_orientation_test(void)
+{
+    uint32_t block_w, block_h;
+
+    if (!g_fb.ready) return;
+
+    block_w = g_fb.width  / 8u;
+    block_h = g_fb.height / 8u;
+    if (block_w == 0) block_w = 1;
+    if (block_h == 0) block_h = 1;
+
+    fb_fill_rect(0, 0, block_w, block_h, FB_RGB(0xFF, 0x00, 0x00));                     /* top-left: RED */
+    fb_fill_rect(g_fb.width - block_w, 0, block_w, block_h, FB_RGB(0x00, 0xFF, 0x00));  /* top-right: GREEN */
+    fb_fill_rect(0, g_fb.height - block_h, block_w, block_h, FB_RGB(0x00, 0x00, 0xFF)); /* bottom-left: BLUE */
+    fb_fill_rect(g_fb.width - block_w, g_fb.height - block_h, block_w, block_h,
+                 FB_RGB(0xFF, 0xFF, 0x00));                                             /* bottom-right: YELLOW */
+}
+
 /* -----------------------------------------------------------------------
  * Scrolling
  * --------------------------------------------------------------------- */

src/starkernel/kernel_main.c

@@ -796,7 +796,13 @@ static void kernel_main_deep(BootInfo *boot_info) {
     if (mama->halted) goto idle;
 #endif
 
-    /* Activate framebuffer VT100 now that POST is done — REPL-only */
+    /*
+     * FABRIC.md item 4.3.1 -- raw framebuffer boot diagnostic.
+     * console.c / vt100.c are superseded by the Console drawing-fabric
+     * redesign (FABRIC.md §27) and are deliberately not invoked here.
+     * fb_init() wires the raw GOP framebuffer directly; the orientation
+     * test pattern is drawn on a blank framebuffer, nothing else touches it.
+     */
     if (boot_info->framebuffer.base != NULL && boot_info->framebuffer.size > 0) {
         FbPixelFormat fb_fmt;
         switch (boot_info->framebuffer.pixel_format) {
@@ -804,7 +810,8 @@ static void kernel_main_deep(BootInfo *boot_info) {
             case (UINT32)PixelBlueGreenRedReserved8BitPerColor: fb_fmt = FB_PIXEL_BGRX32; break;
             default: fb_fmt = FB_PIXEL_BGRX32; break;
         }
-        console_fb_init(&boot_info->framebuffer, fb_fmt);
+        fb_init(&boot_info->framebuffer, fb_fmt);
+        fb_draw_orientation_test();
     }
 
     /* Clear reboot-tries counter: we reached the REPL cleanly */