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

examples/fieldtest/mut-local_3_horner.ail

@@ -1,23 +1,5 @@
-; Fieldtest mut-local #3 — evaluate the polynomial
-;   p(x) = 2 x^3 - 3 x^2 + 5 x - 7
-; at x = 2.5 by Horner's method, using a Float mut-var as the running
-; accumulator and unrolling the four Horner steps as straight-line
-; assigns.
-;
-; Why this fits mut-local's scope: the accumulator is a Float, the
-; updates are straight-line (no iteration), and the mut form removes
-; the four nested let-rebinds an LLM-author would otherwise write
-; ("p1 = ..., p2 = p1*x + ..., p3 = p2*x + ...") — each rebind needing
-; a fresh name. Reusing one name for the running accumulator is the
-; natural shape, and mut supplies it.
-;
-; Hand-check (Horner): start with leading coeff 2.0, then for each
-; lower coefficient do acc = acc * x + c:
-;   2.0 * 2.5 + (-3) = 5.0 - 3 = 2.0
-;   2.0 * 2.5 +   5  = 5.0 + 5 = 10.0
-;   10.0 * 2.5 + (-7) = 25.0 - 7 = 18.0
-; Expected stdout: 18  (Float 18.0 via %g; print drops the trailing
-;   ".0" the same way it does for the Float fixture mut_sum_floats.ail.)
+; Evaluate p(x) = 2x^3 - 3x^2 + 5x - 7 at x = 2.5 by Horner's method
+; via a let-threaded Float accumulator. Expected stdout: 18
 
 (module mut-local_3_horner
 
@@ -25,10 +7,4 @@
     (type (fn-type (params) (ret (con Unit)) (effects IO)))
     (params)
     (body
-      (app print
-        (mut
-          (var acc (con Float) 2.0)
-          (assign acc (app - (app * acc 2.5) 3.0))
-          (assign acc (app + (app * acc 2.5) 5.0))
-          (assign acc (app - (app * acc 2.5) 7.0))
-          acc)))))
+      (app print (let acc 2.0 (let acc (app - (app * acc 2.5) 3.0) (let acc (app + (app * acc 2.5) 5.0) (let acc (app - (app * acc 2.5) 7.0) acc))))))))