feat(source): expose borrowed read-only mixed source eight validation

docs/language.rst

@@ -6,6 +6,26 @@ Arcadia Studio OA / SA Language and Editing Contract
 :Core implementation: ``crates/oa-core``
 :Text suffix: ``.arcadia``
 
+Staged source8 Rust read API
+----------------------------------------
+
+The core library separately offers ``language::check_mixed_bundle(&SourceBundle,
+ProjectLimits)`` for complete ordinary/typed source8 validation. Its opaque
+``CheckedMixed`` result borrows the unchanged input and exposes only ``project()``
+and ``warnings()``. Explicit UUIDs are required for the project, every ordinary
+and typed declaration, and typed parameters; no IDs are generated. All supplied
+canonical files must be reachable from the entry. Errors return no admitted
+model; success preserves actual located warnings and the complete model.
+
+This bounded API selects both retained text and entry producer checks and carries
+the caller's whole-project policy through admission. It does not promise an
+all-heap memory bound. It performs no filesystem discovery, migration, formatting,
+source stabilization, indexing, saving or history installation. Existing ordinary
+parse/compile/update APIs, CLI commands, desktop and server flows remain separate;
+a read result does not grant their typed editing or save authority. Generic and
+typed depth policies and platform qualification limits remain unchanged. This
+staged capability does not replace the broader conformance obligations below.
+
 1. Scope and conformance
 ------------------------