source: scrub iter-code / Decision-N residue from inline comments

Follow-up to bcd4181: the remaining ~530 inline `//` and `///`
comments still carrying opaque shorthand are now reformulated to
their content phrases. The only surviving `iter-<code>` reference
in source is the literal filename
`docs/journals/2026-05-13-iter-mq.3.md` (a real journal file).

Sweep covered:
  - `// Iter X.Y: <text>` prefixes (Iter 13a / 14a / 14e / 15g-aux /
    16b.x / 16d / 16e / 18b / 18c.x / 18d.x / 18e / 18g.x / 19a /
    19a.1 / 19b / 20a / 20f / 22-floats.x / 22b.x / 22c / 23.x /
    24.1 / cli-diag-human / hs.x / str-concat / etc.) — fully
    removed; the descriptive text that followed each prefix stays.
  - `// (Decision N)` and `per Decision N` and `Decision N axis 3` —
    replaced with the content phrase plus the relevant contract
    file (`design/contracts/tail-calls.md` for Decision 8,
    `design/contracts/memory-model.md` for Decision 10,
    `design/contracts/typeclasses.md` and `design/models/typeclasses.md`
    for Decision 11, `design/contracts/authoring-surface.md` for
    Decision 6, "the transitional dual-allocator" for Decision 9,
    "Effect prose" for Decision 3).
  - `// mq.X / mq.X (Task N) / mq.X journal / mq.X invariant` ->
    "the canonical-class-form rule / Class-class repurpose /
    method-dispatch-refactor journal / canonical-class-form
    invariant".
  - `// ct.X / ct.X (canonical-type-names) / ct.1.5a + ctt.2 /
    ct.2 Task N` -> "the canonical-form rule for type references /
    the canonical-form normalisation step / canonical-type-lookup
    refactor".
  - `// eob.X` -> "heap-Str-ABI" / "the Str carve-out".
  - `// rpe.X` -> "the per-type-print-op retirement".
  - `// post-mq.X / Pre-ct.X / pre-mq.X` -> "post-canonical-class-form" /
    "Pre-canonical-type-form" etc.
  - `/// Iter X regression: / /// Iter X.Y: / /// Iter A arm-close /
    /// Iter ct.4 (...): / /// Iter rpe.1 ...` -> descriptive
    phrases.

The journal filename in `crates/ailang-core/src/workspace.rs:573`
stays verbatim because it points at an actual file under
`docs/journals/`.

Tests: full `cargo test --workspace` green (80/80 test-result blocks
clean, no FAILED line). design_index_pin 5/5 + docs_honesty_pin 5/5
gating tests pass.

crates/ailang-codegen/src/subst.rs

@@ -15,7 +15,7 @@ use std::collections::{BTreeMap, BTreeSet};
 
 use super::{CodegenError, Result};
 
-/// Iter 12b: derive a name → concrete-type substitution from the
+/// derive a name → concrete-type substitution from the
 /// declared params of a `Forall` body and the actual arg types at a
 /// call site. Walks both sides in parallel; whenever a `Type::Var`
 /// (rigid name) appears on the params side, binds it to the
@@ -45,7 +45,7 @@ pub(crate) fn derive_substitution(
     // through return-type unification, but at the call site we only
     // see args; if needed, callers can extend this with expected-ret
     // info.
-    // Iter 15a: a forall var that the args couldn't pin (e.g.
+    // a forall var that the args couldn't pin (e.g.
     // `is_none(Nothing) : forall a. (Maybe a) -> Bool` — `a` is
     // genuinely unobservable from the args alone) defaults to `Unit`.
     // The specialised body must not actually read an `a`-typed value,
@@ -71,7 +71,7 @@ pub(crate) fn unify_for_subst(
     vars: &BTreeSet<&str>,
     subst: &mut BTreeMap<String, Type>,
 ) -> Result<()> {
-    // Iter 14a fix, extended in 15g-aux: a `$u`-prefixed var is a
+    // A `$u`-prefixed var is a
     // synth-only wildcard produced by `synth_arg_type` for nullary
     // ctors of a parameterised ADT (e.g. `Nil : List<$u>`). It
     // carries no real constraint — accept without binding so a
@@ -104,7 +104,7 @@ pub(crate) fn unify_for_subst(
     match (param, arg) {
         (Type::Var { name }, _) if vars.contains(name.as_str()) => {
             if let Some(prev) = subst.get(name).cloned() {
-                // Iter 15b: the previously-bound type may be more
+                // the previously-bound type may be more
                 // concrete than `arg` (e.g. `prev = List<Int>` from a
                 // sibling binding, `arg = List<$u>` from a synth-
                 // wildcard nullary ctor). Use recursive unification
@@ -149,7 +149,7 @@ pub(crate) fn unify_for_subst(
     }
 }
 
-/// Iter 15a: rewrites bare `Type::Con` references that resolve against
+/// rewrites bare `Type::Con` references that resolve against
 /// `owner_local_types` into qualified `module.Type` form. Mirrors
 /// `ailang_check::qualify_local_types`. Used when the codegen pulls a
 /// polymorphic fn signature across the import boundary; without this
@@ -204,7 +204,7 @@ pub(crate) fn qualify_local_types_codegen(
     }
 }
 
-/// Iter 12b: substitute rigid type vars in `t` according to `subst`.
+/// substitute rigid type vars in `t` according to `subst`.
 /// Used to specialise the type of a polymorphic def for a given
 /// instantiation.
 pub(crate) fn apply_subst_to_type(t: &Type, subst: &BTreeMap<String, Type>) -> Type {