boot: separate BIOS from UEFI

boot/bios/ now holds the MBR, the El Torito stub and the real-mode stage
2; boot/uefi/ is empty and documented.

Separate directories rather than #ifdefs, because the two share almost
nothing. A BIOS loader lives in 16-bit real mode, reaches the firmware
through software interrupts, and walks itself up into long mode. A UEFI
loader is a 64-bit PE that the firmware has already placed in long mode
with paging on, calling services through a table of function pointers.
What they do share is the hand-off contract, which boot/README.md now
writes down: ELF segments placed, long mode, a 24-byte-entry memory map
at E820_MAP_ADDR, framebuffer geometry at VBE_INFO_ADDR.

That contract is the point of documenting it now. UEFI's GetMemoryMap
returns EFI_MEMORY_DESCRIPTORs, and the right move is to translate them
into the entries kernel/arch/x86_64/e820.c already parses rather than
teach the kernel a second format.

Build output paths under $(BUILD)/boot are unchanged; only sources moved.

.claude/memory/schunix-iso-boot.md

@@ -18,7 +18,7 @@ protected mode before it reads the kernel. That rules out both El Torito
 emulation modes (hard-disk emulation exists only through the BIOS) and floppy
 emulation (2.88 MB cap). So:
 
-- `boot/cdboot.S` — a separate stage 1 for optical media. `tools/mkiso.sh`
+- `boot/bios/cdboot.S` — a separate stage 1 for optical media. `tools/mkiso.sh`
   passes `-boot-load-size 25`, so the BIOS loads cdboot *and* all 24 sectors of
   stage2 before anything of ours runs, putting stage2 at 0x7E00 exactly where
   `bootasm.S` would have. cdboot only records the medium and jumps.