docs: catch the docs up with the finished package split

The plan doc's status records completion (all waves + phase 3) and its
execution notes describe the finished shape; the fresh-session pointer
names build-support's userBinary instead of the deleted
addUserBinaryImpl, and the size-check carry-along note is discharged.
README gains the build/ directory in the layout tree and splits the
source-map row across root build.zig / build/images.zig /
build/qemu.zig. testing.md points at the distributed per-package test
steps; threading.md, threading-plan.md, driver-model.md, efi.md,
display.md, system-requirements.md, and devices-csv.md's adding-a-
driver checklist stop describing the pre-package build.

docs/system-requirements.md

@@ -108,7 +108,7 @@ hypervisor configured for UEFI firmware and an xHCI USB controller.
 - **UEFI only.** A custom UEFI application loader is installed to
   `\EFI\BOOT\BOOTX64.efi`. There is **no BIOS, multiboot, or limine** path. The
   loader tolerates UEFI Class-3 machines with no legacy PIC/PIT.
-  (`build.zig:246`, `boot/efi.zig`)
+  (`build/images.zig` — the EFI/BOOT install — and `boot/efi.zig`)
 - **ACPI is the hardware-discovery mechanism.** The RSDP is taken from the UEFI
   configuration table (ACPI 2.0 GUID preferred, 1.0 fallback). Without a valid
   RSDP there is **no device discovery** — no SMP, no IOAPIC routing, no PCI/USB.