iter loop-recur.1: additive Term::Loop / Term::Recur / LoopBinder foundation

First of three iterations of the standalone loop/recur milestone
(spec 97c1ed1). loop/recur become real parseable / printable /
round-trippable / hash-stable strictly-additive AST nodes across
every no-wildcard exhaustive Term match in all six crates, plus
parse/print, prose arms, DESIGN.md + form_a.md schema blocks,
drift/coverage/hash anchors, and the spec's worked sum_to program
as a green round-trip fixture. NO typecheck semantics and NO real
codegen this iter by design: synth and lower_term are stubbed
CheckError::Internal / CodegenError::Internal exactly as mut.1
(real typecheck = iter 2, real loop-header/phi/back-edge = iter 3).

Two binding Boss design calls recorded in the plan header and the
journal: (1) verify_tail_positions "byte-unchanged" = its tail-app
verification role is unchanged, not its source — it is an
exhaustive no-wildcard match so two descent-only arms are
mandatory; acceptance evidence is the tail-app non-regression test.
(2) codegen is in iter-1 scope as stubs/pass-throughs because the
workspace must compile for cargo test --workspace.

Three plan-pseudo-vs-reality substitutions (serde Type::Con
literal, bare Int -> (con Int), unqualified LoopBinder) and one
recon-undercount integration-test site, all intent-preserving and
journalled. Boss forward-fix folded in: the committed specs
themselves wrote bare Int in the loop-binder snippets (parses as
Type::Var) — corrected the three headline snippets to (con Int) for
doc-honesty (no design/schema change).

cargo test --workspace 600 -> 608 / 0 red (Boss-reran
independently); iter13a + new loop_recur hash pins green.

crates/ailang-prose/src/lib.rs

@@ -935,6 +935,34 @@ fn write_term_prec(out: &mut String, t: &Term, level: usize, parent_prec: u8, ow
             out.push_str(" := ");
             write_term(out, value, level, owning_module);
         }
+        Term::Loop { binders, body } => {
+            // loop-recur iter 1: minimal-correctness Form-B render.
+            // Prose surface for loop is not yet designed; render the
+            // shape so a reader sees it without overcommitting.
+            out.push_str("loop {\n");
+            for b in binders {
+                indent(out, level + 1);
+                out.push_str(&b.name);
+                out.push_str(" = ");
+                write_term(out, &b.init, level + 1, owning_module);
+                out.push_str(";\n");
+            }
+            indent(out, level + 1);
+            write_term(out, body, level + 1, owning_module);
+            out.push('\n');
+            indent(out, level);
+            out.push('}');
+        }
+        Term::Recur { args } => {
+            out.push_str("recur(");
+            for (i, a) in args.iter().enumerate() {
+                if i > 0 {
+                    out.push_str(", ");
+                }
+                write_term(out, a, level, owning_module);
+            }
+            out.push(')');
+        }
     }
 }
 
@@ -1133,6 +1161,28 @@ fn count_free_var(name: &str, t: &Term) -> usize {
             let n_use = if assign_name == name { 1 } else { 0 };
             n_use + count_free_var(name, value)
         }
+        // loop-recur iter 1: a binder named `name` shadows the outer
+        // binding for later binder inits and the body. Inits before
+        // the shadowing binder still see the outer `name`.
+        Term::Loop { binders, body } => {
+            let mut total = 0usize;
+            let mut shadowed = false;
+            for b in binders {
+                if !shadowed {
+                    total += count_free_var(name, &b.init);
+                }
+                if b.name == name {
+                    shadowed = true;
+                }
+            }
+            if !shadowed {
+                total += count_free_var(name, body);
+            }
+            total
+        }
+        Term::Recur { args } => {
+            args.iter().map(|a| count_free_var(name, a)).sum()
+        }
     }
 }
 
@@ -1285,6 +1335,45 @@ fn subst_var_with_term(t: &Term, name: &str, replacement: &Term) -> Term {
             name: assign_name.clone(),
             value: Box::new(subst_var_with_term(value, name, replacement)),
         },
+        // loop-recur iter 1: lexical-shadow semantics symmetric to
+        // count_free_var — once a binder named `name` is declared,
+        // later inits and the body are not rewritten.
+        Term::Loop { binders, body } => {
+            let mut shadowed = false;
+            let new_binders: Vec<ailang_core::ast::LoopBinder> = binders
+                .iter()
+                .map(|b| {
+                    let init = if shadowed {
+                        b.init.clone()
+                    } else {
+                        subst_var_with_term(&b.init, name, replacement)
+                    };
+                    if b.name == name {
+                        shadowed = true;
+                    }
+                    ailang_core::ast::LoopBinder {
+                        name: b.name.clone(),
+                        ty: b.ty.clone(),
+                        init,
+                    }
+                })
+                .collect();
+            let body_rw = if shadowed {
+                (**body).clone()
+            } else {
+                subst_var_with_term(body, name, replacement)
+            };
+            Term::Loop {
+                binders: new_binders,
+                body: Box::new(body_rw),
+            }
+        }
+        Term::Recur { args } => Term::Recur {
+            args: args
+                .iter()
+                .map(|a| subst_var_with_term(a, name, replacement))
+                .collect(),
+        },
     }
 }