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/driver-model.md

@@ -145,9 +145,8 @@ The build side of this has since landed: the shared recipe in
 device/service clients (`driver`, `block`, `display`, `input`), plus `mmio`,
 `xkeyboard-config`, `acpi-ids` — into every user binary, and per-binary extras —
 protocol modules, bus logic — are added with `programModule(exe).addImport(...)`.
-Most binaries are still built by the root `build.zig`'s stanzas through that recipe;
-a binary can instead own a package with its own ~15-line `build.zig` (pci-bus is the
-first — see [build-packages-plan.md](../build-packages-plan.md)). That's the *entire*
+Every binary owns a package with its own ~15-line `build.zig` calling that recipe
+(see [build-packages-plan.md](../build-packages-plan.md)). That's the *entire*
 mechanism — Zig modules already give you everything else.
 
 The discipline that makes this work: **a class driver must not import a bus's *hardware*