docs: catch the build docs up with the package split

Review findings: the plan doc claimed no implementation existed, had the
domain dependency order wrong (kernel depends on protocol; device on
kernel + protocol + csv), never placed the three shared contracts, and
named a nonexistent new-driver-checklist.md. Its status now records the
implemented phases (and the deliberate pci-bus-first pilot), the target
shape carries the contract placements and the path-dependency-only
constraint on the kernel package's out-of-root abi export, and the
execution notes describe the post-pilot build for whichever session
runs the remaining waves. README's repo layout gains build-support/
and the packages-note; driver-model, threading, system-requirements,
and the two display plan docs stop citing root build.zig for recipe
facts that now live in build-support.

docs/README.md

@@ -263,9 +263,18 @@ test/            → /test      the test tree: the QEMU harness (qemu_test.py, h
   system/services/            beside the on-image test fixtures — vfs-test/ thread-test/
                               crash-test/ … — whose repo path IS their boot-volume path
                               (/test/system/services/<name>)
+build-support/                the danos build API (build-time only, nothing on the image):
+                              the shared user-binary recipe + default-import wiring every
+                              build file consumes (docs/build-packages-plan.md)
 tools/                        host-side build scripts
 ```
 
+**Builds are packages** (docs/build-packages-plan.md): each `library/` domain owns a
+`build.zig`/`build.zig.zon` exporting its modules (with a standalone `zig build test`),
+binaries are converting one directory at a time to ~15-line package builds (`pci-bus`
+is the first), and the root `build.zig` orchestrates — image assembly, QEMU, the
+aggregate test step.
+
 **Wire protocols live in `library/protocol/`**, one module per directory
 (`library/protocol/vfs/vfs-protocol.zig` is the `vfs-protocol` module), imported by module
 name. A protocol is the seam between a low-level driver and the higher-level service it
@@ -320,5 +329,5 @@ exception in [coding-standards.md](coding-standards.md) applies to that seam.
 | System services (init, the `fat` filesystem, the device-manager) | `system/services/` |
 | Device drivers, one sub-project each (`pci-bus`, `ps2-bus`, `usb-xhci-bus` bus drivers) | `system/drivers/` |
 | On-image test fixtures for the QEMU cases (`vfs-test`, `crash-test`, `thread-test`, …) → `/test/system/services` | `test/system/services/` |
-| Build + `run-x86-64` (QEMU/OVMF) + `release-x86-64` (the flashable ISO) | `build.zig` |
+| Build orchestration + `run-x86-64` (QEMU/OVMF) + `release-x86-64` (the flashable ISO) | `build.zig` (root; the shared user-binary recipe is `build-support/`, and each `library/` domain + packaged binary carries its own `build.zig`) |
 | QEMU integration test harness | `test/qemu_test.py` |