iter remove-mut-var-assign.1: atomic removal of mut/var/assign

mut/var/assign removed from AILang entirely and atomically. Deleted:
Term::Mut/Term::Assign/struct MutVar; the three Form-A keywords +
parse_mut/parse_assign + grammar EBNF; the 4 mut CheckError variants;
the mut_scope_stack synth threading (param dropped from synth + every
internal/external/test caller); the two lower_term arms; and every
exhaustive no-_ Term::Mut/Term::Assign match arm across 17 source
files — cut in lockstep with DESIGN.md, fixtures, the drift trio,
carve-out and roadmap so the schema is honest at every commit. No
catch-all wildcard introduced (verified). loop/recur + let/if are
the surviving forms.

The shared codegen alloca machinery survives (loop reuses it):
mut_var_allocas renamed binder_allocas (representation-only, loop
codegen byte-identical) and the shared Term::Lam escape guard
simplified to !loop_stack.is_empty() with the loop half
(LoopBinderCapturedByLambda) byte-equivalent. Feature-acceptance
applied inverted: the removed feature fails clause 2 (redundant)
and clause 3 (IS the iterated-mutable-state bug class).

Behaviour preservation is executable: mut_counter/mut_sum_floats
still print 55 after the faithful let/if rewrite. The removal is
made executable by the new mut_removed_pin.rs (4 must-fail pins).
Independent verification: cargo test --workspace 605/0, zero
residual mut symbols in any crate source, loop/recur non-regression
all green (55 / 500000500000 / infinite-compiles / the
lambda_capturing_loop_binder pin), roundtrip_cli PASS.

One DONE_WITH_CONCERNS: a 4th recurrence of the recon-undercount
class (in-source mod tests + a drift-pin fn + 5 orphaned mut
.ail.json carve-outs + a non-enumerated E0599); all resolved within
implementer remit, no behaviour change. Milestone-close audit then
fieldtest remain.

spec docs/specs/2026-05-18-remove-mut-var-assign.md (grounding PASS)
plan docs/plans/remove-mut-var-assign.1.md

crates/ail/tests/ct1_check_cli.rs

@@ -187,48 +187,23 @@ fn check_ordering_match_post_migration_is_clean() {
     );
 }
 
-/// Property (RED — fieldtest mut-local F2): in non-JSON (human) mode,
-/// the human-stderr formatter prepends the diagnostic code exactly once
-/// as the canonical `[code]` bracket prefix (the cli-diag-human format).
-/// The four mut-local `CheckError` variants
-/// (`mut-assign-out-of-scope`, `assign-type-mismatch`,
-/// `mut-var-unsupported-type`, `mut-var-captured-by-lambda`) must NOT
-/// also embed `[code] ` inside their `thiserror` Display body, which
-/// would render the bracketed code TWICE in the user-visible stderr
-/// line (`error: [code] fn: [code] message`).
+/// Property: in non-JSON (human) mode, the human-stderr formatter
+/// prepends the diagnostic code exactly once as the canonical
+/// `[code]` bracket prefix (the cli-diag-human format). The
+/// `loop-binder-captured-by-lambda` `CheckError` Display body must
+/// NOT also embed `[code] ` inside its `thiserror` Display body,
+/// which would render the bracketed code TWICE in the user-visible
+/// stderr line (`error: [code] fn: [code] message`).
 ///
 /// This pins the *observable* doubling on the real CLI human path
 /// (`crates/ail/src/main.rs` non-JSON `Cmd::Check` arm), not the raw
-/// Display string of one variant in isolation: it counts occurrences
-/// of the literal `[<code>]` token in the rendered stderr and requires
-/// exactly one. Drops to GREEN once the four Display bodies shed their
-/// leading `[<code>] ` prefix (the formatter's prefix then supplies it
-/// once). The non-mut variants are unaffected because they never
-/// embedded the bracket.
+/// Display string of the variant in isolation: it counts occurrences
+/// of the literal `[<code>]` token in the rendered stderr and
+/// requires exactly one (the formatter supplies it; the Display body
+/// must not also embed it).
 #[test]
-fn check_human_mode_renders_mut_diagnostic_code_exactly_once() {
-    // (fixture filename, kebab diagnostic code) for the four mut-local
-    // negative fixtures shipped by the mut-local milestone.
+fn check_human_mode_renders_loop_binder_diagnostic_code_exactly_once() {
     let cases = [
-        (
-            "test_mut_assign_out_of_scope.ail.json",
-            "mut-assign-out-of-scope",
-        ),
-        (
-            "test_mut_assign_type_mismatch.ail.json",
-            "assign-type-mismatch",
-        ),
-        (
-            "test_mut_var_unsupported_type.ail.json",
-            "mut-var-unsupported-type",
-        ),
-        (
-            "test_mut_var_captured_by_lambda.ail.json",
-            "mut-var-captured-by-lambda",
-        ),
-        // Iter loop-recur.tidy: symmetric loop-binder-capture variant.
-        // Its Display body is likewise bracket-`[code]`-free so the
-        // human formatter supplies `[<code>]` exactly once.
         (
             "test_loop_binder_captured_by_lambda.ail.json",
             "loop-binder-captured-by-lambda",