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.

docs/architecture/kernel-history.md

@@ -142,14 +142,14 @@ no room to add so much as a print statement.
 
 Split into two stages, the standard way real bootloaders solve this:
 
-- **Stage 1** (`boot/bootasm.S`, still the literal MBR sector, still
+- **Stage 1** (`boot/bios/bootasm.S`, still the literal MBR sector, still
   ≤510 bytes) does exactly one thing: raw IDE PIO (LBA28) reads
   `STAGE2_SECTORS` (16) sectors starting at disk sector 1 into memory at
   `0x7E00`, then jumps there. Nothing else fits, nothing else is needed.
   (Caught a real bug writing this: `outb $0x1F7,%al`-style immediates
   silently truncate to 8 bits — ports 0x1F2-0x1F7 are all > 0xFF and need
   the `%dx` register form, or you're hitting the wrong I/O port entirely.)
-- **Stage 2** (`boot/stage2.S` + `boot/stage2main.c`, loaded at `0x7E00`,
+- **Stage 2** (`boot/bios/stage2.S` + `boot/bios/stage2main.c`, loaded at `0x7E00`,
   occupies disk sectors 1-16) does everything stage 1 used to skip straight
   to, plus a boot banner (BIOS teletype, real mode, before the VBE mode
   switch — note this only reaches the video device, not serial, so it
@@ -287,7 +287,7 @@ across a long `ls`-heavy session to see what's actually climbing.
 
 ## 6. Boot speed
 
-Two cheap, real wins in `boot/stage2main.c`, no functional changes:
+Two cheap, real wins in `boot/bios/stage2main.c`, no functional changes:
 
 - `progress_tick()` (called once per 512-byte disk sector) used to trigger
   a full progress-bar redraw — border plus fill, a few thousand pixel