build: delete module_homes — imports resolve through the declared zon

The module-to-domain table in build-support duplicated what each
domain's build.zig already states with its addModule exports. userBinary
now resolves each named import by searching the packages the binary
declared in its own build.zig.zon (b.available_deps), which also makes
the zon the literal include path: an import can only be satisfied by a
domain the binary claims, and naming a module whose domain is missing
fails the build graph with the domain to declare. build-support is down
to the recipe alone. All build variants green; manifest unchanged.

docs/build-packages-plan.md

@@ -62,8 +62,10 @@ Rules:
   include list — and its zon names only the domains those modules come from
   (plus `build-support` and `kernel`, which is implicit in every binary: the
   root shim and user link script live there). Nothing is pre-wired: an
-  undeclared `@import` is a compile error, and build-support's one
-  module-to-domain table (`module_homes`) resolves each name. Availability
+  undeclared `@import` is a compile error, and build-support resolves each
+  name by searching the packages the zon declares — the domains' own
+  addModule exports are the single statement of who owns what, with no name
+  table anywhere to drift. Availability
   never meant bloat — Zig only compiles what a program actually imports — but
   exactness makes the declared interface honest and machine-checked.
 - **Modules export source, not artifacts** — each consumer compiles libraries
@@ -132,9 +134,9 @@ rewritten against the package template.
 ## Execution notes (the finished shape)
 
 - The shared recipe lives in `build-support/build.zig`: `userBinary` (what
-  every binary package calls, resolving each named import through the
-  `module_homes` table) and `programModule` (for per-binary addOptions
-  modules). The `start` root shim and `user.ld` are named through the kernel
+  every binary package calls; each named import resolves by searching the
+  packages the binary's zon declares) and `programModule` (for per-binary
+  addOptions modules). The `start` root shim and `user.ld` are named through the kernel
   package (Dependency.path).
 - Adding a binary = adding a directory with source + a ~15-line build.zig +
   zon (copy any existing binary package, e.g.