refactor: home the target facts in a dependency-free std/target.sx

Move OS / ARCH / POINTER_SIZE + the OperatingSystem / Architecture enums out of
build.sx into a new std/target.sx that imports NOTHING, so low-level code can
name the target enum types without dragging the build/std barrel (build.sx
transitively pulls std + compiler + bundle, ~16k lines of IR).

build.sx flat-imports target.sx so the decls stay registered in the standard
import graph (build.sx is reachable from std.sx — dropping it would shift every
std program's type table). This is not a re-export: flat import only splices
into build.sx's own scope. Consumers are unaffected — the compiler resolves
OS / ARCH / POINTER_SIZE by name (comptime constants), so `inline if OS`/value
reads need no import; a module that names the enum type imports target.sx.

No behavior change (full suite green, 817/0); the enum types stay in the same
import graph, so no .ir snapshot drift.

library/modules/build.sx

@@ -8,13 +8,18 @@
 // (comptime-only, never emitted into the binary), and the build↔bundle import
 // cycle resolves like the std↔build one.
 #import "modules/platform/bundle.sx";
-
-OperatingSystem :: enum { macos; linux; windows; wasm; ios; android; unknown; }
-Architecture :: enum { aarch64; x86_64; wasm32; wasm64; unknown; }
-
-OS : OperatingSystem = .unknown;
-ARCH : Architecture = .unknown;
-POINTER_SIZE : i64 = 8;
+// The target facts — OS / ARCH / POINTER_SIZE + the OperatingSystem /
+// Architecture enums — live in a dependency-free module (std/target.sx) so
+// low-level code can name the enum TYPES without the build/std baggage.
+// build.sx imports it (flat) so those decls stay registered in the standard
+// import graph (build.sx is reachable from std.sx — dropping the import would
+// shift every std program's type table). This is NOT a re-export: flat import
+// only splices the names into build.sx's own scope, never into build.sx's
+// importers. Consumers need no import anyway — the compiler resolves OS / ARCH
+// / POINTER_SIZE by name (comptime constants, for both `inline if` and value
+// reads); a module that names OperatingSystem / Architecture imports
+// std/target.sx directly.
+#import "modules/std/target.sx";
 
 // An opaque compile-time build-configuration handle. `build_options()` hands one
 // back; its real state lives on the compiler's threaded `BuildConfig`. The handle