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

docs/language.html

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+<section id="staged-source8-rust-read-api">
+<h2>Staged source8 Rust read API</h2>
+<p>The core library separately offers <span class="docutils literal"><span class="pre">language::check_mixed_bundle(&amp;SourceBundle,</span> ProjectLimits)</span> for complete ordinary/typed source8 validation. Its opaque
+<span class="docutils literal">CheckedMixed</span> result borrows the unchanged input and exposes only <span class="docutils literal">project()</span>
+and <span class="docutils literal">warnings()</span>. 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.</p>
+<p>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.</p>
+</section>
 <section id="scope-and-conformance">
 <h2>1. Scope and conformance</h2>
 <p>This specification defines the shared Operational Analysis (OA) and System