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.

crates/ailang-core/src/hash.rs

@@ -1,14 +1,45 @@
 //! Content-addressed hashing for definitions.
 //!
-//! Hash = BLAKE3 over the canonical JSON form (see `canonical`).
-//! The `hash` field in the input is removed before hashing.
+//! Hash = BLAKE3 over the canonical JSON bytes of a [`Def`] (see
+//! [`crate::canonical`]). The hash function takes a [`Def`] by
+//! reference, so there is no `hash` field to strip — the in-memory
+//! struct does not carry one.
+//!
+//! The single entry point at this level is [`def_hash`]. The parallel
+//! entry point at module granularity is
+//! [`crate::workspace::module_hash`].
 
 use crate::ast::Def;
 use crate::canonical;
 
-/// 16-hex-char (64-bit) prefix of the BLAKE3 hash.
-/// Enough for uniqueness within realistic codebases and compact
-/// enough for visual inspection.
+/// Content hash of a single [`Def`] — the 16-hex-char (64-bit) prefix
+/// of its BLAKE3 hash over canonical JSON bytes.
+///
+/// 64 bits is wide enough to be unique across realistic AILang
+/// codebases and short enough to read at a glance in pretty-printed
+/// manifests. The hash is computed over the **canonical JSON byte
+/// pre-image**, not over the in-memory struct, so any change to the
+/// canonical form (new fields, different `skip_serializing_if`
+/// behaviour, key reordering bug) changes every hash. The Iter 13a
+/// regression test below pins concrete hashes for two example
+/// definitions to catch that.
+///
+/// # Examples
+///
+/// ```ignore
+/// use ailang_core::{ast::*, def_hash};
+///
+/// let def = Def::Const(ConstDef {
+///     name: "answer".into(),
+///     ty: Type::int(),
+///     value: Term::Lit { lit: Literal::Int { value: 42 } },
+///     doc: None,
+/// });
+///
+/// // Stable across runs: same canonical bytes -> same hash.
+/// assert_eq!(def_hash(&def), def_hash(&def));
+/// assert_eq!(def_hash(&def).len(), 16);
+/// ```
 pub fn def_hash(def: &Def) -> String {
     let bytes = canonical::to_bytes(def);
     let h = blake3::hash(&bytes);