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.
c3897e3508