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/scoreboard.md

@@ -1,12 +1,12 @@
 # Go-on-SX Scoreboard
 
-**Total: 370 / 370 tests passing**
+**Total: 377 / 377 tests passing**
 
 |  | Suite | Pass | Total |
 |---|---|---|---|
 | ✅ | lex | 129 | 129 |
 | ✅ | parse | 176 | 176 |
-| ✅ | types | 65 | 65 |
+| ✅ | types | 72 | 72 |
 | ⬜ | eval | 0 | 0 |
 | ⬜ | runtime | 0 | 0 |
 | ⬜ | stdlib | 0 | 0 |