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/testing.md

@@ -12,7 +12,9 @@ There are two layers:
   `system/abi.zig`, `library/device/model/device-abi.zig`) now spans ~26 modules:
   protocol and on-wire definitions (VFS, USB, virtio-gpu), the FAT engine, the
   display compositor, PS/2 and HID decoding, the kernel log ring, and the
-  runtime's `time`/`thread` — the full list is the test step in `build.zig`.
+  runtime's `time`/`thread` — the list is distributed across the library-domain
+  and binary packages' own `test` steps, which the root `zig build test`
+  aggregates (docs/build-packages-plan.md).
   These compile for the host and run natively.
 - **QEMU integration tests** (`python3 test/qemu_test.py`) — boot the real kernel
   and check its behaviour. This is the interesting part.