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/std/target.sx

@@ -0,0 +1,23 @@
+// modules/std/target.sx — compiler-injected target facts. Dependency-free
+// (imports NOTHING) so any low-level module can read the target enums/values
+// without pulling the std barrel.
+//
+// The compiler sets OS / ARCH / POINTER_SIZE per build target (name-matched in
+// src/imports.zig). NOTE: a top-level `inline if OS/ARCH/POINTER_SIZE`
+// conditional is resolved by the compiler's flatten pre-pass and needs NO
+// import at all (that is how std/net/epoll.sx selects its layout). Import this
+// module only when you need the enum TYPE (`OperatingSystem` / `Architecture`)
+// or a value in a non-`inline if` position.
+//
+// build.sx imports this (flat) so the decls stay registered in the standard
+// import graph — NOT to re-export them (flat import doesn't propagate to a
+// module's importers). Code that names the OperatingSystem / Architecture TYPE
+// imports this module directly; OS / ARCH / POINTER_SIZE values and `inline if`
+// conditions are resolved by the compiler by name and need no import.
+
+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;