WhatsNew + roadmap: mut-local milestone closed; Stateful-islands marked in-progress

- docs/WhatsNew.md: append a user-facing entry describing the new
  local-mutable-state block — what it does, what it does not yet
  cover (heap-Str and user ADTs), the two example fixtures, and
  the broader streaming-workloads motivation. No internals.

- docs/roadmap.md: the Stateful-islands milestone flips from [ ] to
  [~] (in progress). A Progress paragraph records sub-milestone 1
  (mut-local) as closed with its commit references, and enumerates
  the remaining sub-milestones (effect-handler infrastructure,
  refs + !Mut, MutArray, Stateful + pipe) — each to be brainstormed
  separately so each one stays small and audit-friendly.

docs/roadmap.md

@@ -265,7 +265,7 @@ clean. Pick the next milestone from P2.)_
   exercising the LLM-natural Show-body shape.
   - context: per-iter journal 2026-05-13-iter-str-concat.md.
 
-- [ ] **\[milestone\]** Stateful islands — bounded mutation for
+- [~] **\[milestone\]** Stateful islands — bounded mutation for
   streaming workloads (`Stateful a b` + `!Mut` effect + `mut`
   syntactic block). Adds a sealed mutable-state layer on top of
   the pure core: a `Stateful a b` first-class type whose interior
@@ -281,6 +281,22 @@ clean. Pick the next milestone from P2.)_
   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). Remaining sub-milestones (to be brainstormed
+  separately, in order): (2) effect-handler infrastructure as a
+  prerequisite for any non-IO/non-Diverge effect; (3) `!Mut` effect
+  + `ref a` first-class type that can escape mut blocks under
+  uniqueness-tracking; (4) mutable-array primitive `MutArray a`;
+  (5) `Stateful a b` sealed callable type + `pipe` combinator with
+  zero-allocation composition + `runST`-equivalent escape discharge.
+
   **Motivation.** AILang's signature-as-contract thesis is *better*
   served by an explicit `Stateful a b` + `!Mut` annotation than by
   the implicit-closure-mutation idiom of myc / Lua / JS factory