smalltalk: Exception/on:do:/ensure:/ifCurtailed: + 15 tests

plans/smalltalk-on-sx.md

@@ -84,7 +84,7 @@ Core mapping:
 - [x] `Object>>respondsTo:`, `Object>>isKindOf:`, `Object>>isMemberOf:`. Universal in `st-primitive-send`. `respondsTo:` searches user method dicts (instance- or class-side based on receiver kind); native primitive selectors aren't enumerated, documented limitation. `isKindOf:` walks `st-class-inherits-from?`; `isMemberOf:` is exact class equality. 26 new tests in `reflection.sx`.
 - [x] `Behavior>>compile:` — runtime method addition. Class-side `compile:` parses the source via `st-parse-method` and installs via `st-class-add-method!`. Sister forms `compile:classified:` and `compile:notifying:` ignore the extra arg (Pharo-tolerant). Returns the selector as a symbol. Also added `addSelector:withMethod:` (raw AST install) and `removeSelector:`. 9 new tests in `reflection.sx`.
 - [x] `Object>>becomeForward:` — one-way become at the universal `st-primitive-send` layer. Mutates the receiver's `:class` and `:ivars` to match the target via `dict-set!`; every existing reference to the receiver dict now behaves as the target. Receiver and target remain distinct dicts (no SX-level identity merge), but method dispatch, ivar reads, and aliases all switch — Pharo's practical guarantee. 6 tests in `reflection.sx`, including the alias case (`a` and `alias := a` both see the new identity).
-- [ ] Exceptions: `Exception`, `Error`, `signal`, `signal:`, `on:do:`, `ensure:`, `ifCurtailed:` — built on top of SX `handler-bind`/`raise`
+- [x] Exceptions: `Exception`, `Error`, `ZeroDivide`, `MessageNotUnderstood` in bootstrap. `signal` raises the receiver via SX `raise`; `signal:` sets `messageText` first. `on:do:` / `ensure:` / `ifCurtailed:` on BlockClosure use SX `guard`. The auto-reraise pattern uses a side-effect predicate (cleanup runs in the predicate, returns false → guard auto-reraises) because `(raise c)` from inside a guard handler hits a known SX issue with nested-handler frames. 15 tests in `lib/smalltalk/tests/exceptions.sx`. Phase 4 complete.
 
 ### Phase 5 — collections + numeric tower
 - [ ] `SequenceableCollection`/`OrderedCollection`/`Array`/`String`/`Symbol`
@@ -108,6 +108,7 @@ Core mapping:
 
 _Newest first. Agent appends on every commit._
 
+- 2026-04-25: Exception system + 15 tests (`lib/smalltalk/tests/exceptions.sx`). Exception/Error/ZeroDivide/MessageNotUnderstood in bootstrap; signal/signal: raise via SX `raise`; on:do:/ensure:/ifCurtailed: on BlockClosure via SX `guard`. Phase 4 complete. 495/495 total.
 - 2026-04-25: `Object>>becomeForward:` + 6 tests. In-place mutation of `:class` and `:ivars` via `dict-set!`; aliases see the new identity. 480/480 total.
 - 2026-04-25: `Behavior>>compile:` + sisters + 9 tests. Parses source via `st-parse-method`, installs via runtime helpers; also added `addSelector:withMethod:` and `removeSelector:`. 474/474 total.
 - 2026-04-25: `respondsTo:` / `isKindOf:` / `isMemberOf:` + 26 tests. Universal at `st-primitive-send`. 465/465 total.