Iter 13d: rustdoc polish for ailang-core + new docwriter agent

Adds ailang-docwriter to /agents/ — a recurring role for keeping
crate-, module-, and pub-item-level rustdoc accurate. First mission:
ailang-core. Crate root, every module root, every pub item documented;
intra-doc links throughout; Iter-13a additions (TypeDef.vars, Type::Con.args)
get an explicit backwards-compat note. Two stale broken-link warnings in
ailang-check fixed in passing. cargo doc --no-deps now warning-free across
the workspace; promoted to verification invariant 6 in DESIGN.md.

docs/DESIGN.md

@@ -241,6 +241,9 @@ ail run <module>                 — build + execute (tempdir), passthrough exit
 4. **Hash stability**: a test ensures the same def always produces the same
    hash.
 5. **CI pin** of the outputs in `tests/expected/`.
+6. **Rustdoc cleanliness**: `cargo doc --no-deps` runs warning-free.
+   Maintained by the `ailang-docwriter` agent (Iter 13d onward); fixing a
+   rustdoc warning is part of the iter that introduced it, not a follow-up.
 
 ## What is not (yet) supported