go: types.sx — interface satisfaction (structural method-set check) + 7 tests [shapes-static-types-bidirectional]

Phase 3 cont. The headline Go-distinguishing typing feature: interfaces
are satisfied *structurally and silently* — no `implements` declaration,
no nominal subtyping. Any type whose method set contains all the
interface's methods (with matching signatures) satisfies it.

Method declarations now type-check via go-check-method-decl:

  * Receiver type extracted (T or *T → "T") via go-extract-recv-ty-name.
  * Method signature (:ty-func PARAMS RESULTS) bound under a mangled
    key "#method/RECV-NAME/METHOD-NAME" in ctx.
  * Body checked with receiver + params extended into the body ctx.

go-iface-satisfies? CTX TY-NAME IFACE-TYPE walks the interface's
:method elements; for each, looks up #method/TY-NAME/METHOD-NAME and
compares (PARAMS, RESULTS) tuples. Embedded interfaces (:embed
elements) skipped in v0 — recursive interface resolution later.

Tests:
  * method-decl binds under #method/Point/String
  * pointer-receiver method also keys the base type
  * Point with String() satisfies interface { String() string }
  * empty type does NOT satisfy Stringer
  * arity-mismatch method fails satisfaction
  * multi-method satisfaction works
  * partial method-set fails

types 72/72, total 377/377. Phase 3 sub-deliverable list is now
substantially complete; only AST-path error context remains as a UX
sharpener.

Sister-plan static-types-bidirectional diary updated with the
**constraint-satisfies? pluggable predicate** kit-API proposal —
third pluggable point after synth/check + assignable?. Go interfaces,
Haskell typeclasses, Rust traits, and TS structural subtyping all
answer "does this value-type fit this constraint-type?" with
different machinery; the kit's check uses constraint-satisfies? when
EXPECTED is itself a constraint type.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

lib/go/types.sx

@@ -531,8 +531,103 @@
         (go-ctx-extend ctx name ty))
       (and (list? decl) (= (first decl) :func-decl))
       (go-check-func-decl ctx decl)
+      (and (list? decl) (= (first decl) :method-decl))
+      (go-check-method-decl ctx decl)
       :else ctx)))
 
+;; ── method declarations and interface satisfaction ──────────────
+;; Methods are recorded in CTX under a mangled key
+;;   "#method/RECV-TYPE-NAME/METHOD-NAME"
+;; bound to the method's :ty-func signature. Interface satisfaction is
+;; a structural lookup over these keys (Go spec § Interface types:
+;; "anything with the matching method set satisfies the interface").
+
+(define
+  go-method-key
+  (fn (recv-ty-name method-name)
+    (str "#method/" recv-ty-name "/" method-name)))
+
+(define
+  go-extract-recv-ty-name
+  ;; Receiver type is T or *T; return the named type's name string.
+  (fn (recv-ty)
+    (cond
+      (and (list? recv-ty) (= (first recv-ty) :ty-name))
+      (nth recv-ty 1)
+      (and (list? recv-ty) (= (first recv-ty) :ty-ptr))
+      (go-extract-recv-ty-name (nth recv-ty 1))
+      :else nil)))
+
+(define
+  go-check-method-decl
+  ;; (list :method-decl RECV NAME PARAMS RESULTS BODY)
+  ;; Binds the method under the mangled key, then checks body with
+  ;; receiver + params extended.
+  (fn (ctx decl)
+    (let ((recv (nth decl 1)) (name (nth decl 2))
+          (params (nth decl 3)) (results (nth decl 4))
+          (body (nth decl 5)))
+      (let ((recv-ty (nth recv 2)))
+        (let ((recv-name (go-extract-recv-ty-name recv-ty)))
+          (let ((sig (list :ty-func
+                       (go-decl-params-to-ty-list params) results)))
+            (let ((ctx2
+                    (cond
+                      (= recv-name nil) ctx
+                      :else
+                      (go-ctx-extend ctx
+                        (go-method-key recv-name name) sig))))
+              (cond
+                (= body nil) ctx2
+                (and (list? body) (= (first body) :block))
+                (let ((body-ctx
+                        (go-extend-with-params
+                          (go-ctx-extend-field ctx2 recv) params)))
+                  (let ((err
+                          (go-check-block body-ctx
+                            (nth body 1) results)))
+                    (cond
+                      (go-type-error? err) err
+                      :else ctx2)))
+                :else ctx2))))))))
+
+(define
+  go-iface-elems-satisfied?
+  ;; Each :method element in ELEMS must have a matching method in CTX
+  ;; under #method/TY-NAME/M-NAME. :embed elements are skipped in v0
+  ;; (they'd need recursive interface resolution).
+  (fn (ctx ty-name elems)
+    (cond
+      (= (len elems) 0) true
+      :else
+      (let ((e (first elems)))
+        (cond
+          (= (first e) :method)
+          (let ((m-name (nth e 1)) (m-params (nth e 2))
+                (m-results (nth e 3)))
+            (let ((found (go-ctx-lookup ctx
+                           (go-method-key ty-name m-name))))
+              (cond
+                (= found nil) false
+                (and (= (nth found 1) m-params)
+                     (= (nth found 2) m-results))
+                (go-iface-elems-satisfied? ctx ty-name (rest elems))
+                :else false)))
+          (= (first e) :embed)
+          (go-iface-elems-satisfied? ctx ty-name (rest elems))
+          :else
+          (go-iface-elems-satisfied? ctx ty-name (rest elems)))))))
+
+(define
+  go-iface-satisfies?
+  ;; Does the type named TY-NAME satisfy the interface IFACE-TYPE
+  ;; under context CTX? Structural method-set match per Go spec.
+  (fn (ctx ty-name iface-type)
+    (cond
+      (not (and (list? iface-type) (= (first iface-type) :ty-interface)))
+      false
+      :else (go-iface-elems-satisfied? ctx ty-name (nth iface-type 1)))))
+
 ;; ── function-decl checking ──────────────────────────────────────
 
 (define