spec: match special form — ADT constructor pattern matching (20 tests)

Extends match-pattern in spec/evaluator.sx with an ADT case: when the
pattern is (CtorName var...) and the value is an ADT dict (:_adt true),
check :_ctor matches, arity matches, then recursively bind field patterns.
Supports nested patterns, wildcard _, variable binding, and zero-arg ctors.

Changes step-sf-match to route no-clause errors through raise-eval-frame
instead of direct error, allowing guard to catch non-exhaustive matches.

40/40 ADT tests pass (20 define-type + 20 match). Zero regressions.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

plans/agent-briefings/primitives-loop.md

@@ -182,7 +182,7 @@ simulate sum types. A native `define-type` + `match` form eliminates this everyw
       `(define-type Name (Ctor1 field...) (Ctor2 field...) ...)`
       Creates constructor functions `Ctor1`, `Ctor2` + predicate `Name?`.
 
-- [ ] Spec: implement `match` special form:
+- [x] Spec: implement `match` special form:
       `(match expr ((Ctor1 a b) body) ((Ctor2 x) body) (else body))`
       Exhaustiveness warning if not all constructors covered and no `else`.
 
@@ -686,6 +686,7 @@ Brief each language's loop agent (or do inline) after rebasing their branch onto
 
 _Newest first._
 
+- 2026-04-26: Phase 6 Spec match done — ADT case added to match-pattern in spec/evaluator.sx: checks (list? pattern)+(symbol? first)+(dict? value)+(get value :_adt), then matches :_ctor+arity and recursively binds field patterns. No-clause error now uses make-cek-value+raise-eval-frame so guard can catch it. 20 new match tests pass; 40/40 total ADT tests green. Zero regressions.
 - 2026-04-26: Phase 6 Spec define-type done — sf-define-type registered via register-special-form! in spec/evaluator.sx; AdtValue as {:_adt true :_type "..." :_ctor "..." :_fields (list ...)}; ctor fns + arity checking + Name?/Ctor? predicates + Ctor-field accessors; *adt-registry* dict populated per define-type call. 20/20 JS tests pass in spec/tests/test-adt.sx. OCaml define-type is next task.
 - 2026-04-26: Phase 6 Design done — plans/designs/sx-adt.md written. Covers define-type/match syntax, AdtValue CEK runtime, stepSfDefineType+MatchFrame dispatch, exhaustiveness warnings, recursive types, nested patterns, wildcard _. 3-phase impl plan. Next fire: Spec implement define-type.
 - 2026-04-26: Phase 5 complete — string buffer fully landed (d98b5fa2). 17 tests, 17/17 OCaml+JS. Phase 6 (ADTs) next.