Iter 16b.1 — local recursive let (no-capture)

Add `(let-rec NAME (params ...) (type ...) (body ...) (in ...))`
as a form-A surface and a `Term::LetRec` AST variant. The 16a
desugar pass lifts each LetRec whose body has no captures from
the enclosing scope to a synthetic top-level fn `<hint>$lr_N`
and substitutes the original name; typecheck and codegen never
see LetRec. Capture detection panics at desugar time, queued
for 16b.2.

Tests: 95 → 99 (+1 e2e local_rec_factorial_demo, +2 desugar
unit, +1 parse unit). The new fixture `examples/local_rec_demo`
runs `fact` at n=1, 3, 5 → prints 1, 6, 120.

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

crates/ailang-core/src/ast.rs

@@ -219,6 +219,22 @@ pub enum Term {
         value: Box<Term>,
         body: Box<Term>,
     },
+    /// Local recursive let-binding (Iter 16b.1). Always fn-shaped:
+    /// the bound name `name` is recursively visible inside `body`.
+    /// Eliminated by `crate::desugar` before typecheck — lifted to a
+    /// synthetic top-level fn when `body` does not capture any name
+    /// from the enclosing lexical scope. The on-disk schema gains the
+    /// `"t": "letrec"` tag; pre-existing fixtures hash bit-identically
+    /// because the variant is additive.
+    LetRec {
+        name: String,
+        #[serde(rename = "type")]
+        ty: Type,
+        params: Vec<String>,
+        body: Box<Term>,
+        #[serde(rename = "in")]
+        in_term: Box<Term>,
+    },
     /// If-expression. Both branches must have the same type.
     If {
         cond: Box<Term>,