docs: fix rustdoc warnings and enforce full API doc coverage

- Wrap in backticks every bare type in rustdoc comments (vertex.rs,
  frame.rs, pipeline_cache.rs, material.rs, mesh.rs, scene.rs, error.rs)
  so rustdoc no longer misreads them as intra-doc links or HTML tags.
- Add a missing doc comment on the public Frame struct.
- Add #![warn(missing_docs)] to the crate root so unevidenced public
  items are surfaced going forward.

cargo doc --no-deps now generates with zero warnings.

lib/src/lib.rs

@@ -25,6 +25,9 @@
 //! use wsg_lib::utils::BASIC_SHADER;
 //! ```
 
+// Warn if a public API item has no rustdoc comment, keeping API coverage at 100%.
+#![warn(missing_docs)]
+
 pub mod app;
 pub mod core;
 pub mod handler;