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/ailang-core/tests/schema_coverage.rs

@@ -47,8 +47,6 @@ enum VariantTag {
     TermSeq,
     TermClone,
     TermReuseAs,
-    TermMut,
-    TermAssign,
     TermLoop,
     TermRecur,
     // Pattern
@@ -96,8 +94,6 @@ const EXPECTED_VARIANTS: &[VariantTag] = &[
     VariantTag::TermSeq,
     VariantTag::TermClone,
     VariantTag::TermReuseAs,
-    VariantTag::TermMut,
-    VariantTag::TermAssign,
     VariantTag::TermLoop,
     VariantTag::TermRecur,
     VariantTag::PatternWild,
@@ -236,18 +232,6 @@ fn visit_term(t: &Term, observed: &mut HashSet<VariantTag>) {
             visit_term(source, observed);
             visit_term(body, observed);
         }
-        Term::Mut { vars, body } => {
-            observed.insert(VariantTag::TermMut);
-            for v in vars {
-                visit_type(&v.ty, observed);
-                visit_term(&v.init, observed);
-            }
-            visit_term(body, observed);
-        }
-        Term::Assign { value, .. } => {
-            observed.insert(VariantTag::TermAssign);
-            visit_term(value, observed);
-        }
         Term::Loop { binders, body } => {
             observed.insert(VariantTag::TermLoop);
             for b in binders {