feat(video): Add a framebuffer console so the TTY and nano work after graphics mode

Pressing Esc left the desktop and started the shell, but nothing appeared in the
emulator window -- the session was only visible on the host terminal. Once the
VBE adapter is switched into graphics mode the VGA text buffer at 0xB8000 stops
being scanned out, so every printk after that point reached serial and the
dmesg ring but never the display.

Disabling the DISPI interface does not bring the text console back; QEMU keeps
presenting the last framebuffer surface. So rather than clawing the adapter back
to text mode, the console is now rendered into the framebuffer:

  drivers/video/fbcon.c draws the console with the shared 8x8 font and carries a
  VT100 subset -- SGR colours, cursor addressing, relative moves, erase-display,
  erase-line and save/restore. It presents a fixed 80x25 grid, because that is
  the geometry the userland assumes (nano pads its bars with %-80s, the shell
  wraps at TTY_WIDTH), with the glyph cell scaled to fill the active resolution.

The TTY driver became the console dispatcher: tty_write, tty_clear and
tty_set_cursor route to fbcon while it owns the display. That last one is what
makes nano work -- it positions with tty_set_cursor() rather than escapes, so
its full-screen repaint was landing as an append-only stream.

Also in this change:

  - `video=WIDTHxHEIGHT` on the kernel command line selects the display mode,
    defaulting to 1280x720 so a 16:9 host panel is filled rather than pillarboxed
    when QEMU goes fullscreen. Rejects modes the heap cannot back.
  - `make run-fullscreen`, plus zoom-to-fit on the windowed target, so scaling
    preserves the guest aspect ratio instead of stretching it.
  - QEMU targets raise vgamem_mb, since a 1280x720 32-bit surface exceeds the
    default VGA memory.
  - bochs_vbe_disable()/fb_set_active() let the framebuffer be marked dormant so
    drawing routines no-op after a mode teardown.

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

Makefile

@@ -264,7 +264,7 @@ ifeq ($(CONFIG_DRV_CANVAS_2D), y)
     CONFIG_SRCS-y += drivers/video/canvas.c
 endif
 ifeq ($(CONFIG_DRV_BOCHS_VBE), y)
-    CONFIG_SRCS-y += drivers/video/bochs.c drivers/video/fb.c
+    CONFIG_SRCS-y += drivers/video/bochs.c drivers/video/fb.c drivers/video/fbcon.c
 endif
 ifeq ($(CONFIG_DRV_SOUND_HDA), y)
     CONFIG_SRCS-y += sound/hda.c sound/sound.c sound/pcm.c sound/tts.c sound/melody.c
@@ -320,7 +320,7 @@ DEPS = $(C_OBJS:.o=.d) $(CPP_OBJS:.o=.d) $(S_OBJS:.o=.d)
 
 .DEFAULT_GOAL := all
 
-.PHONY: all clean run debug info configure menuconfig config tui nconfig defconfig x86_64_defconfig aarch64_defconfig armv8_defconfig armv8i_defconfig armv81_defconfig arm32_defconfig qemu help
+.PHONY: all clean run run-fullscreen debug info configure menuconfig config tui nconfig defconfig x86_64_defconfig aarch64_defconfig armv8_defconfig armv8i_defconfig armv81_defconfig arm32_defconfig qemu help
 
 all: $(TARGET)
 
@@ -414,23 +414,39 @@ $(IMAGE): $(BUILD_DIR)/boot/boot.bin $(BUILD_DIR)/boot/stage2.bin $(BUILD_DIR)/k
 run qemu: $(TARGET)
 	$(QEMU_CMD)
 
+# -vga std gives the Bochs VBE adapter the kernel drives. vgamem_mb is raised
+# because a 1280x720 32-bit framebuffer needs more than the 8 MB default.
+QEMU_VGA = -device VGA,vgamem_mb=32
+
 run-gui: $(TARGET)
 ifeq ($(ARCH), x86_64)
 	qemu-system-x86_64 -drive format=raw,file=$(IMAGE) \
 		-netdev user,id=net0,hostfwd=tcp::2222-:22,hostfwd=tcp::8080-:80 \
-		-device e1000,netdev=net0 -vga std -serial stdio
+		-device e1000,netdev=net0 $(QEMU_VGA) -display gtk,zoom-to-fit=on -serial stdio
 else ifneq ($(filter $(ARCH), aarch64 arm64),)
 	qemu-system-aarch64 -M virt -cpu cortex-a57 -m 128M -kernel $(BUILD_DIR)/kernel.elf -device virtio-gpu-pci -serial stdio
 else
 	qemu-system-arm -M virt -cpu cortex-a15 -m 128M -kernel $(BUILD_DIR)/kernel.elf -device virtio-gpu-pci -serial stdio
 endif
 
+# Fullscreen without distortion: zoom-to-fit scales the guest while preserving
+# its aspect ratio, letterboxing rather than stretching on a wider panel.
+run-fullscreen: $(TARGET)
+ifeq ($(ARCH), x86_64)
+	qemu-system-x86_64 -drive format=raw,file=$(IMAGE) \
+		-netdev user,id=net0,hostfwd=tcp::2222-:22,hostfwd=tcp::8080-:80 \
+		-device e1000,netdev=net0 $(QEMU_VGA) \
+		-display gtk,zoom-to-fit=on,show-cursor=on -full-screen -serial stdio
+else
+	@echo "run-fullscreen targets x86_64 only"
+endif
+
 run-vnc: $(TARGET)
 ifeq ($(ARCH), x86_64)
 	@echo "=== [QEMU VNC Server running at localhost:5900 (VNC Display :0)] ==="
 	qemu-system-x86_64 -drive format=raw,file=$(IMAGE) \
 		-netdev user,id=net0,hostfwd=tcp::2222-:22,hostfwd=tcp::8080-:80 \
-		-device e1000,netdev=net0 -vga std -vnc :0 -serial stdio
+		-device e1000,netdev=net0 $(QEMU_VGA) -vnc :0 -serial stdio
 else
 	@echo "=== [QEMU VNC Server running at localhost:5900 (VNC Display :0)] ==="
 	qemu-system-aarch64 -M virt -cpu cortex-a57 -m 128M -kernel $(BUILD_DIR)/kernel.elf -device virtio-gpu-pci -vnc :0 -serial stdio