build: lazy dependencies — a build loads only what it ships

The 13 /test fixtures and the acpi/fdt discovery pair are .lazy in the
root zon, resolved with b.lazyDependency only when a build actually
bundles them: a plain `zig build` neither compiles the fixtures nor
loads their build files, and only the -Ddiscovery-selected package ever
loads. Fixture packages are uniform (dependency name = artifact name =
boot-path leaf), so the bundled list shrinks to a name loop. Production
manifest byte-identical; the -Dtest-case manifest carries the same 13
entries in the same order; -Ddiscovery=fdt exercises the lazy fdt path.
Discharges the plan's deferred what-this-buys #4.

docs/build-packages-plan.md

@@ -9,7 +9,9 @@ named "discovery" that the root's -Ddiscovery picks between), and phase 3 (the
 root split into `build/images.zig` + `build/qemu.zig`; the root `build.zig` is
 ~460 lines of orchestration, down from ~1,250). Every phase landed green: unit
 tests, the QEMU suite at parity with main, boot-image file list unchanged.
-Still future: `lazyDependency` for image-specific builds (What-this-buys #4).
+The `lazyDependency` payoff (What-this-buys #4) is in too: the /test fixtures
+and the unselected discovery package are lazy — a build loads and compiles
+only what it ships.
 
 ## Why
 
@@ -76,7 +78,9 @@ Rules:
    stability testing in isolation.
 3. Adding a binary = adding a directory (source + two small files), not editing
    three places in a 1,250-line file.
-4. Later: `lazyDependency` lets an image target build only what it ships.
+4. `lazyDependency` lets an image target build only what it ships: the /test
+   fixtures resolve only under -Dtest-case, and only the -Ddiscovery-selected
+   discovery package ever loads.
 
 ## Phases