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the boot path, since vt100.c/console.c are superseded by the Console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.

include/starkernel/framebuffer.h

@@ -84,6 +84,17 @@ void fb_fill_rect(uint32_t x, uint32_t y, uint32_t w, uint32_t h,
 void fb_draw_glyph(uint32_t px, uint32_t py, uint8_t ch,
                    uint32_t fg, uint32_t bg);
 
+/* -----------------------------------------------------------------------
+ * Boot diagnostic
+ * --------------------------------------------------------------------- */
+
+/**
+ * One-time boot diagnostic (FABRIC.md item 4.3.1): fills each raster corner
+ * with a distinct solid color so a screendump reveals orientation. Not part
+ * of the Console drawing fabric -- diagnostic-only.
+ */
+void fb_draw_orientation_test(void);
+
 /* -----------------------------------------------------------------------
  * Scrolling
  * --------------------------------------------------------------------- */