Iter 13e: rustdoc polish for ailang-check

Second docwriter mission. Pure rustdoc additions (no API or
behaviour change) across the typechecker crate:

- builtins.rs: module root expanded; EffectOpSig (struct + 3
  fields) and install() got /// strings.
- diagnostic.rs: Severity (+ both variants), Diagnostic (+
  severity/code/message fields) and the error/with_def/with_ctx
  helpers got /// strings; super:: link rewritten to crate::.
- lib.rs: CheckError + every variant, to_diagnostic, CheckedModule
  (+ symbols), check, Env (+ globals/effect_ops/types/
  module_globals/current_module), CtorRef (+ type_name) got ///
  strings; crate-root prose upgraded with intra-doc link to
  check_module.

Verification: cargo doc --no-deps zero warnings; cargo build
--workspace green; cargo test --workspace 64/64 + 3 ignored
doctests green. Diff is 188 LOC, all in /// or //! lines (verified
by filtering).

Findings (not fixed; orchestrator-deferred):
- Env is pub but only privately constructable.
- CheckError::CtorArity and ::ArityMismatch share the public
  diagnostic code "arity-mismatch" by design.
- Diagnostic / Severity reachable via two paths because the
  diagnostic module is pub.

JOURNAL.md updated with the Iter 13e entry. 13f (ailang-codegen
+ ail) and 14a (List a rewrite) remain queued.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/JOURNAL.md

@@ -1242,3 +1242,73 @@ After 13d there's no urgency on 13e/f — `cargo doc` is already
 warning-free. They're "polish iterations to be slotted in
 between feature iters when context budget is short". 14a
 (`List a` rewrite) remains the next-feature default.
+
+## Iter 13e — rustdoc polish for `ailang-check`
+
+Second docwriter mission. Same mandate as 13d, applied to the
+typechecker crate. `lib.rs` (1922 LOC) had a strong crate root
+already — covers HM-with-effects, top-level forall, the
+rigid-vs-metavar split, the `$m<id>` encoding — but its `pub`
+surface (the `Env` struct, `CheckError` + 24 variants,
+`CheckedModule`, `CtorRef`, the `check_module` entry point) was
+mostly undocumented. `builtins.rs` had a one-line module
+header and zero `///` strings on `EffectOpSig` or `install`.
+`diagnostic.rs` had a strong module header (lists every stable
+diagnostic code) but `Diagnostic`, `Severity`, and the
+construction helpers were undocumented.
+
+Agent added 188 LOC of pure rustdoc across the three files; no
+non-doc lines changed (verified by filtering the diff). Each
+`CheckError` variant now carries the AST term/type that
+triggers it and the stable kebab-case `code` it maps to. `Env`
+fields (`globals`, `effect_ops`, `types`, `module_globals`,
+`current_module`) got individual `///` strings that name the
+invariants — most importantly that `module_globals` includes
+the current module, which the agent flagged as undocumented at
+field level (now fixed). Crate-root prose got an upgraded
+intra-doc link to `[`check_module`]`; `super::` reference in
+`diagnostic.rs` rewritten to `crate::` (cosmetic, but the
+canonical form).
+
+**Findings reported** (judgement deferred to me):
+
+- `Env` is `pub` with all-`pub` fields but `Env::new` is
+  private and there's no public builder — external callers can
+  only construct via field-by-field literal, which is fragile
+  if a field is added later. **Not fixing**: changing this is
+  an API decision, not a doc one. Worth raising next time we
+  touch the crate's public surface deliberately.
+- `CheckError::CtorArity` and `CheckError::ArityMismatch` both
+  serialize to the public diagnostic code `arity-mismatch`. The
+  `///` strings now flag the collision per-variant; tooling
+  that consumes the diagnostic JSON sees only the merged code
+  and that's intentional from the iter-5b vintage. **Not
+  fixing.**
+- `Diagnostic` and `Severity` are reachable both via the crate
+  re-export and via `crate::diagnostic::*` because the
+  `diagnostic` module is itself `pub`. Rustdoc renders both
+  pages; harmless but slightly noisy. **Not fixing**: the
+  re-export is the documented entry point and we don't want to
+  hide the module.
+
+**Process note.** Same shape as 13d: I wrote a brief naming
+the deficiencies (numbers of pub items, which files had thin
+roots), the agent did the doc additions inside its mandate,
+the orchestrator-side work was authoring DESIGN/JOURNAL and
+verifying the diff. Cost ≈ 32 tool uses for 188 LOC of doc
+across 3 files — denser than 13d (more sentences per pub item
+because the typechecker invariants need explicit
+articulation), but the per-LOC payoff for a future reader is
+also higher.
+
+**Tests:** 64/64 unit + e2e (unchanged), 3 ignored doctests
+(unchanged). `cargo doc --no-deps`: 0 warnings (was 0; the
+agent introduced 4 transient broken-link warnings during the
+work and resolved all of them before reporting done). `cargo
+build --workspace` green. `cargo test --workspace` green.
+
+**Plan 13f / 14a unchanged.** 13f (`ailang-codegen` + `ail`
+CLI) is the natural next polish iter; 14a (`List a` rewrite of
+`list_map`) remains the next-feature default. Auto-mode is on,
+so I'll continue into 13f directly unless context budget
+pressures a switch.