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/codegen_import_map_fallback_pin.rs

@@ -91,20 +91,9 @@ fn synthesised_print_uses_user_module_show_via_fallback() {
             Term::ReuseAs { source, body } => {
                 contains_xmod_show_var(source) || contains_xmod_show_var(body)
             }
-            // Iter mut.1: a `Term::Mut` cannot itself host a
-            // `show_user_adt.show__<T>` reference (its body is a
-            // mut-block, not a synthesised polymorphic call), but
-            // recurse defensively so any nested reference inside a
-            // var init / body / assign-value still surfaces.
-            Term::Mut { vars, body } => {
-                vars.iter().any(|v| contains_xmod_show_var(&v.init))
-                    || contains_xmod_show_var(body)
-            }
-            Term::Assign { value, .. } => contains_xmod_show_var(value),
             // loop-recur iter 1: a `Term::Loop` cannot itself host a
             // synthesised cross-module reference, but recurse
-            // defensively through binder inits / body / recur args,
-            // mirroring the `Term::Mut` arm above.
+            // defensively through binder inits / body / recur args.
             Term::Loop { binders, body } => {
                 binders.iter().any(|b| contains_xmod_show_var(&b.init))
                     || contains_xmod_show_var(body)