build: Track header dependencies, fix default goal, and repair the font table comment

Three build-correctness fixes plus README coverage of the new kernel core.

The Makefile had no header dependency tracking, so editing a header left every
unrebuilt caller linked against the old layout. That is how the desktop's
window paint callbacks silently landed at the wrong struct offset after
gui_window_t grew: gui_apps.o was never recompiled. Compiles now emit .d files
via -MMD -MP and the build pulls them in.

The include has to sit at the end of the file with .DEFAULT_GOAL pinned to
`all` -- a .d file's first rule would otherwise become make's default goal and
turn a bare `make` into a silent no-op.

drivers/video/canvas.c carried the same font table defect gui_gfx.c did: the
`// \` comment on the backslash glyph continues onto the next line, swallowing
the `]` entry and shifting every glyph from ']' onward by one, so all lowercase
text rendered one letter off.

Co-Authored-By: Claude Opus 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01L5yKJT4Wo2cV3rfPrwY5MP

drivers/video/canvas.c

@@ -70,7 +70,7 @@ static const uint8_t font8x8_basic[96][8] = {
     {0x66,0x66,0x66,0x3c,0x18,0x18,0x3c,0x00}, // Y
     {0xfe,0x06,0x0c,0x18,0x30,0x60,0xfe,0x00}, // Z
     {0x3c,0x30,0x30,0x30,0x30,0x30,0x3c,0x00}, // [
-    {0xc0,0x60,0x30,0x18,0x0c,0x06,0x02,0x00}, // \
+    {0xc0,0x60,0x30,0x18,0x0c,0x06,0x02,0x00}, // backslash
     {0x3c,0x0c,0x0c,0x0c,0x0c,0x0c,0x3c,0x00}, // ]
     {0x10,0x38,0x6c,0xc6,0x00,0x00,0x00,0x00}, // ^
     {0x00,0x00,0x00,0x00,0x00,0x00,0x00,0xff}, // _