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/device-driver-development/devices-csv.md

@@ -96,7 +96,12 @@ in different namespaces — against the right `bus` column.
 
 ## Adding a driver
 
-1. Build the driver binary and bundle it at `/system/drivers/<name>` (build.zig).
+1. Create `system/drivers/<name>/` with the driver source plus a ~15-line
+   package `build.zig` + `build.zig.zon` (copy an existing driver package,
+   e.g. `system/drivers/pci-bus/`; per-driver extras go through
+   `build_support.programModule`). Then bundle it at `/system/drivers/<name>`:
+   one dependency + one bundled entry in the root `build.zig`, one line in the
+   root `build.zig.zon`.
 2. Add a row to `etc/devices.csv` naming the identity it binds and its full path.
 
 No device-manager change is required — the registry is the seam.