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/os-development/efi.md

@@ -24,8 +24,9 @@ EFI/BOOT/BOOTX64.efi         <- the "removable media" default for x86-64
 ```
 
 The boot volume is **FHS-shaped** (see the repository-layout note in
-[README.md](../README.md)): `build.zig` installs `boot/efi.zig` (built for the `uefi`
-target) at `EFI/BOOT/BOOTX64.efi` — the one path UEFI firmware fixes — and lays
+[README.md](../README.md)): the root `build.zig` compiles `boot/efi.zig` (built
+for the `uefi` target) and `build/images.zig` places it at
+`EFI/BOOT/BOOTX64.efi` — the one path UEFI firmware fixes — and lays
 the rest out by FHS path: the kernel at `system/kernel`, init at
 `system/services/init`, the pre-packed boot capsule at `boot/system.img`
 ([system-image.md](system-image.md)).

docs/os-development/threading-plan.md

@@ -22,11 +22,12 @@ lands on its own and ends in a **verifiable gate** — shaped for a `/loop` run,
 ## Conventions
 
 Follow [coding-standards.md](../coding-standards.md): spell out non-acronym abbreviations,
-kebab-case file names, no `Co-Authored-By` trailers. New user binaries go through
-`addUserBinary` (with the new `threaded` flag where a binary spawns threads) and get
-packed into the initial-ramdisk; new syscalls extend [abi.zig](../../system/abi.zig)
-`SystemCall` + a `library/runtime` wrapper; test services live beside the code they
-exercise and register a `ServiceId` if they must be looked up.
+kebab-case file names, no `Co-Authored-By` trailers. New user binaries are
+packages whose build.zig calls `build_support.userBinary` (with `.threaded =
+true` where a binary spawns threads) and get packed into the initial-ramdisk;
+new syscalls extend [abi.zig](../../system/abi.zig) `SystemCall` + a
+`library/kernel` wrapper; test services live beside the code they exercise and
+register a `ServiceId` if they must be looked up.
 
 ## How to verify along the way
 

docs/os-development/threading.md

@@ -237,9 +237,9 @@ see the intro). Two scoped pieces, as built:
 
 ### Build: multi-threaded codegen, opt-in
 
-A binary opts in with `addThreadedUserBinary` in the root `build.zig` (or
-`.threaded = true` in a binary package's `build_support.userBinary` call) — the
-shared recipe in build-support then builds it `single_threaded = false` — so atomics
+A binary opts in with `.threaded = true` in its package's
+`build_support.userBinary` call — the shared recipe in build-support then builds it
+`single_threaded = false` — so atomics
 and (later) TLS are real. Threads and atomics are unsound in a `single_threaded` image,
 so a binary must opt in **before** it may call `Thread.spawn`. Everyone else
 stays single-threaded and lean.