Implement floppy disk controller driver (AI)

Add Intel 82077AA-compatible floppy disk controller driver with:
- CMOS-based drive detection (register 0x10)
- FDC reset with DOR toggle and SPECIFY command
- Motor control with spin-up delay
- ISA DMA channel 2 setup for data transfers
- LBA-to-CHS conversion for 1.44MB geometry
- Single-sector read/write via DMA with 7-byte result phase
- Seek and recalibrate with sense interrupt verification
- IRQ 6 handler (vector 38) for command completion
- Devicefs integration as 'floppy' class block devices

The IRQ wait function detects whether interrupts are enabled
(EFLAGS IF bit) and temporarily enables them if needed, allowing
the driver to work during early init before the kernel calls STI.
The scheduler safely handles timer interrupts in this window since
no user processes exist yet.

Tested with QEMU: drive detected as 1.44M 3.5", registered as
/dev/floppy1, full boot succeeds with CD+HDD+floppy attached.

build.log

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+[  3%] Building user-mode applications
+Building app: cat
+  Built: /workspaces/claude-os/build/apps_bin/cat (310 bytes)
+Building app: env-test
+/usr/bin/ld: warning: /workspaces/claude-os/build/apps_bin/env-test.elf has a LOAD segment with RWX permissions
+  Built: /workspaces/claude-os/build/apps_bin/env-test (389 bytes)
+Building app: fork-test
+  Built: /workspaces/claude-os/build/apps_bin/fork-test (132 bytes)
+Building app: hello-world
+  Built: /workspaces/claude-os/build/apps_bin/hello-world (49 bytes)
+Building app: ls
+  Built: /workspaces/claude-os/build/apps_bin/ls (250 bytes)
+Building app: mount
+  Built: /workspaces/claude-os/build/apps_bin/mount (992 bytes)
+Building app: sh
+/workspaces/claude-os/apps/sh/sh.c:167:17: warning: unused variable 'type' [-Wunused-variable]
+  167 |         int32_t type = readdir(resolved, 0, name);
+      |                 ^~~~
+1 warning generated.
+  Built: /workspaces/claude-os/build/apps_bin/sh (3428 bytes)
+[  3%] Built target apps
+[  6%] Built target initrd
+[  9%] Building C object src/CMakeFiles/kernel.dir/floppy.c.o
+[ 12%] Linking C executable ../bin/kernel
+[ 96%] Built target kernel
+[100%] Generating bootable ISO image
+xorriso 1.5.6 : RockRidge filesystem manipulator, libburnia project.
+
+Drive current: -outdev 'stdio:/workspaces/claude-os/release/claude-os.iso'
+Media current: stdio file, overwriteable
+Media status : is blank
+Media summary: 0 sessions, 0 data blocks, 0 data,  127g free
+Added to ISO image: directory '/'='/tmp/grub.oPDKGd'
+xorriso : UPDATE :     581 files added in 1 seconds
+Added to ISO image: directory '/'='/workspaces/claude-os/build/isodir'
+xorriso : UPDATE :     586 files added in 1 seconds
+xorriso : NOTE : Copying to System Area: 512 bytes from file '/usr/lib/grub/i386-pc/boot_hybrid.img'
+xorriso : UPDATE : Thank you for being patient. Working since 0 seconds.
+ISO image produced: 5918 sectors
+Written to medium : 5918 sectors at LBA 0
+Writing to 'stdio:/workspaces/claude-os/release/claude-os.iso' completed successfully.
+
+[100%] Built target iso