P2.1: domain structs + boundary validation

Add the core distribution domain model under src/domain/ (App, Release,
Artifact, Channel, AuditEvent + Platform/Visibility/ValidationStatus/
RolloutPolicy enums) and a boundary validator that returns one distinct
typed ValidationErr per failure class (BadSlug, EmptyVersion, BadVersion,
BadChannelName, UnknownPlatform, MissingField, BadRollout).

Pure sx, depends only on modules/std.sx; lookups left as linear scans over
List (no HashMap). tests/domain_validate.sx asserts valid App/Release/
Artifact/Channel are accepted and each invalid case is rejected with the
exact expected error tag.

src/domain/platform.sx

@@ -0,0 +1,11 @@
+#import "modules/std.sx";
+
+// Target platforms a release can ship to. `android_apk` names the APK
+// artifact form specifically; the remaining variants are one per OS.
+Platform :: enum u8 {
+    ios;
+    android_apk;
+    macos;
+    linux;
+    windows;
+}