iter prose-loop-binders.1: Form-B loop binders as parenthesised init-list

The Term::Loop arm of write_term rendered binders as bare
`name = init;` statements inside the `loop { … }` body block, which
reads as C/Rust re-init-every-iteration semantics — a projection
lying about a body the prose surface exists to let the reader trust.
Rewrite to a parenthesised init-list on the keyword line,
`loop(acc = 0, i = 1) { … }`, positionally isomorphic to the
unchanged Term::Recur arm. Projection-only: loop/recur AST, Form-A,
JSON-AST, typecheck, codegen, and the Form-A↔JSON round-trip
invariant untouched. RED-first via two new committed
examples/*.prose.txt byte-equality snapshots + two snapshot_loop_*
tests; full ailang-prose suite 10/0; both ail-prose CLI byte-matches
green. Single-iteration milestone, brainstorm→plan→implement.

Spec: docs/specs/2026-05-18-prose-loop-binders.md
Plan: docs/plans/prose-loop-binders.1.md

examples/loop_forever_build.prose.txt

@@ -0,0 +1,14 @@
+// module loop_forever_build
+
+/// loop-recur iter 3 — an infinite loop (no non-recur exit) must COMPILE
+/// (typechecks AND compiles; no termination claim). Build-only; by design never
+/// returns, so the binary is never executed.
+fn main() -> Unit with IO {
+  print(spin(0))
+}
+
+fn spin(n: Int) -> Int {
+  loop(i = 0) {
+    recur(i + 1)
+  }
+}

examples/loop_sum_to_run.prose.txt

@@ -0,0 +1,16 @@
+// module loop_sum_to_run
+
+/// loop-recur iter 3 — sum 1..10 via loop/recur. Expected stdout: 55.
+fn main() -> Unit with IO {
+  print(sum_to(10))
+}
+
+fn sum_to(n: Int) -> Int {
+  loop(acc = 0, i = 1) {
+    if i > n {
+      acc
+    } else {
+      recur(acc + i, i + 1)
+    }
+  }
+}