fix(codegen): resolve the dotted T.new user-ADT callee, symmetric to check

A monomorphic `(new Counter 42)` over a user-defined ADT that declares a
`new` fn passed `ail check` but crashed `ail build --alloc=rc` with
`unknown variable: Counter.new` (RED-pinned in 1eff055).

Root: the `Term::New` desugar lowers a no-type-arg monomorphic
`(new Counter 42)` to `(app Counter.new 42)` — a type-scoped `Var`
callee `Counter.new`. The checker resolves that dotted callee via its
TypeDef-first ladder (so check is clean), but codegen had no equivalent
resolution: the name was absent from the import map and the current
module's def list, so it fell through to `CodegenError::UnknownVar`. The
user `new` fn body IS codegen'd — only the dotted call-site failed to
resolve. The error surfaced first in the arg-type pre-pass
(`synth_with_extras`'s dotted-Var branch), which `lower_term`'s
`Term::Let` arm runs before lowering the call.

Fix (crates/ailang-codegen/src/lib.rs, all `// #53`):
- New `Emitter::type_home_module(type_name)`: returns the module
  declaring the named TypeDef (current module first, then any import
  target) by scanning `module_ctor_index` for a matching
  `CtorRef.type_name`. This mirrors the checker's TypeDef-first ladder.
- A dotted `T.def` fallback (resolve via the type's home module) added
  to the three sites that needed it: `resolve_top_level_fn` (the
  fn-pointer / lower_app resolution path), `is_static_callee` (its
  lockstep partner), and the `synth_with_extras` dotted-Var branch (the
  path that actually fired for this fixture).

Lockstep (CLAUDE.md `lower_app` ↔ `is_static_callee`): `is_static_callee`
now returns true for a dotted `T.def` whose `T` is a user ADT declared
in this or an imported module, exactly matching `resolve_top_level_fn`'s
new fallback — so no name is claimed static without being lowerable.

Distinct root from #51 (the polymorphic `(new T <elem> …)` type-arg
drop, ee4107c) — that fix deliberately left the monomorphic
app-lowering path unchanged; this completes it.

Verification: the #53 pin (crates/ail/tests/user_adt_new_mono_pin.rs)
is green; the original 3-module reproducer
examples/fieldtest/kem_2_counter_new.ail builds, runs, prints 42;
ailang-codegen/check/core suites, the intercepts bijection, and both
hash-pins are green; full workspace green.

closes #53

crates/ailang-codegen/src/lib.rs

@@ -2547,11 +2547,23 @@ impl<'a> Emitter<'a> {
             let target_module = match self.import_map.get(prefix).cloned() {
                 Some(m) => m,
                 None if self.module_user_fns.contains_key(prefix) => prefix.to_string(),
-                None => {
-                    return Err(CodegenError::Internal(format!(
-                        "cross-module call `{name}`: prefix `{prefix}` not in import map"
-                    )))
-                }
+                // #53: type-scoped `T.def` callee (e.g. `Counter.new`,
+                // produced by the monomorphic `Term::New` desugar). When
+                // `prefix` is not an import alias or module name but
+                // names a TypeDef in scope, the call resolves to the
+                // `def` fn in that type's home module — symmetric to the
+                // checker's TypeDef-first ladder. Keeps the documented
+                // `lower_app` ↔ `is_static_callee` lockstep honest:
+                // `is_static_callee` already returns true for every
+                // single-dot name, so this branch MUST resolve it.
+                None => match self.type_home_module(prefix) {
+                    Some(home) => home,
+                    None => {
+                        return Err(CodegenError::Internal(format!(
+                            "cross-module call `{name}`: prefix `{prefix}` not in import map"
+                        )))
+                    }
+                },
             };
             // iter 23.4: codegen-time poly-call dispatch removed. Post-mono
             // every poly call site has been rewritten by `rewrite_mono_calls`
@@ -2786,6 +2798,30 @@ impl<'a> Emitter<'a> {
     }
 
 
+    /// #53: returns the module that declares the named TypeDef — the
+    /// current module first, then imported modules. `None` if no module
+    /// in scope declares a TypeDef of that name. A type's presence in a
+    /// module is read from `module_ctor_index` (a TypeDef declares at
+    /// least one ctor, each `CtorRef` carrying its `type_name`). This
+    /// mirrors the checker's TypeDef-first ladder so codegen resolves a
+    /// type-scoped dotted callee (`T.def`) exactly where the checker
+    /// does. Used by `lower_app` and `resolve_top_level_fn`.
+    fn type_home_module(&self, type_name: &str) -> Option<String> {
+        if let Some(ctors) = self.module_ctor_index.get(self.module_name) {
+            if ctors.values().any(|c| c.type_name == type_name) {
+                return Some(self.module_name.to_string());
+            }
+        }
+        for target in self.import_map.values() {
+            if let Some(ctors) = self.module_ctor_index.get(target) {
+                if ctors.values().any(|c| c.type_name == type_name) {
+                    return Some(target.clone());
+                }
+            }
+        }
+        None
+    }
+
     /// is `name` a callee that can be resolved at compile time
     /// (no fn-pointer needed)? True for builtin operators, the
     /// current-module top-level fns (mono or poly), and qualified
@@ -2872,11 +2908,17 @@ impl<'a> Emitter<'a> {
             // a valid post-mono construct because both ends were
             // independently typechecked under their original module
             // contexts before mono ran).
-            let target = match self.import_map.get(prefix) {
-                Some(m) => m,
-                None => prefix,
+            // #53: a type-scoped `T.def` prefix (e.g. `Counter.new`)
+            // resolves to the type's home module, symmetric to the
+            // checker's TypeDef-first ladder and the matching arm in
+            // `lower_app`'s cross-module branch. Tried after import_map
+            // and the literal-module-name fallback.
+            let target: String = match self.import_map.get(prefix) {
+                Some(m) => m.clone(),
+                None if self.module_user_fns.contains_key(prefix) => prefix.to_string(),
+                None => self.type_home_module(prefix).unwrap_or_else(|| prefix.to_string()),
             };
-            let sig = self.module_user_fns.get(target)?.get(suffix)?.clone();
+            let sig = self.module_user_fns.get(&target)?.get(suffix)?.clone();
             return Some((format!("@ail_{target}_{suffix}_clos"), sig));
         }
         if let Some(sig) = self
@@ -3234,6 +3276,14 @@ impl<'a> Emitter<'a> {
                     // post-mono synthesised cross-module references
                     // (see `resolve_top_level_fn` and the cross-module
                     // call arm in `lower_app` for matching fallbacks).
+                    // #53: a type-scoped `T.def` prefix (e.g. `Counter.new`,
+                    // the monomorphic `Term::New` desugar output) resolves
+                    // to the type's home module, after import_map and the
+                    // literal-module-name fallback. Mirrors the checker's
+                    // TypeDef-first ladder and the matching arms in
+                    // `lower_app` / `resolve_top_level_fn`, so arg-type
+                    // synthesis of the dotted callee doesn't hit `UnknownVar`.
+                    let type_home = self.type_home_module(prefix);
                     let target_opt: Option<&str> = self
                         .import_map
                         .get(prefix)
@@ -3244,7 +3294,8 @@ impl<'a> Emitter<'a> {
                             } else {
                                 None
                             }
-                        });
+                        })
+                        .or(type_home.as_deref());
                     if let Some(target) = target_opt {
                         if let Some(ty) = self
                             .module_def_ail_types