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

docs/roadmap.md

@@ -45,9 +45,12 @@ work progresses.
   window, JSON tags fail closed as generic unknown-variant, codegen
   alloca machinery kept (loop reuses it) + renamed `binder_allocas`,
   shared `Term::Lam` escape guard keeps its `loop` half. Explicitly
-  decoupled from Stateful-islands (separate, deferred). Spec written
-  + grounding-check PASS; awaiting user spec review → planner.
-  - context: `docs/specs/2026-05-18-remove-mut-var-assign.md`
+  decoupled from Stateful-islands (separate, deferred). Spec
+  approved + planned + implemented (605/0, loop/recur
+  non-regression green, removal made executable); milestone-close
+  `audit` then `fieldtest` (surface-touching) remain.
+  - context: `docs/specs/2026-05-18-remove-mut-var-assign.md`;
+    `docs/journals/2026-05-18-iter-remove-mut-var-assign.1.md`
 
 - [x] **\[milestone\]** Prose `loop` binders — projection redesign —
   CLOSED 2026-05-18. Form-B prose now renders loop binders as a
@@ -434,16 +437,6 @@ work progresses.
   zero-allocation pipe combinator, growing tuple-state types as
   pipelines lengthen).
 
-  **Progress.** Decomposed at brainstorm time (2026-05-15) into a
-  sequence of shippable sub-milestones. Sub-milestone 1 closed
-  2026-05-15: **mut-local** — sealed-by-construction `mut`/`var`/
-  `assign` blocks for Int/Float/Bool/Unit scalars, alloca-resident,
-  no escape, no `!Mut` effect leakage. Foundation in place;
-  fn signatures stay pure while LLM authors can write imperative
-  accumulators directly. Spec `docs/specs/2026-05-15-mut-local.md`;
-  iters mut.1/mut.2/mut.3/mut.4-tidy (commits 7b92719, b24718a,
-  03fb633, 20add51).
-
   **Sub-milestone sequencing is UNDER RE-THINK with the user
   (2026-05-16) — no further sub-milestone is planned or brainstormed
   until that conversation happens.** Why: the 2026-05-15 decomposition