go: types.sx scaffold — synth/check skeleton + 12 tests; Phase 3 starts [shapes-static-types-bidirectional]

First slice of Phase 3 (bidirectional type checker).

lib/go/types.sx defines:
  * go-ctx-empty / go-ctx-extend / go-ctx-lookup — context as a value.
  * go-ctx-extend-field — consumes the (:field NAMES TYPE) shape from
    the parser, binding every name to the shared type. This is the
    cross-deliverable validation of the :field binding-group
    observation made during Phase 2 func decls: parser produces it,
    type checker consumes it, same shape end-to-end.
  * go-predeclared — true / false / nil baked in. Full list expanded
    on demand.
  * go-synth — currently handles variable lookup; literals / calls /
    binops follow in subsequent iterations.
  * go-check — v0 defers to synth + structural type equality. Untyped-
    constant flow and assignment-compatibility relations land later.
  * Type errors carry first-class tags (:unbound, :mismatch,
    :unsupported-synth) so consumers and tooling can dispatch.

Conformance.sh wired with new types suite. Scoreboard cleanup: drop
the "pending" types row since the suite is now real.

types 12/12, total 317/317. Phase 3 underway.

Sister-plan static-types-bidirectional diary updated with the
synth/check shape: judgment skeleton, error tag structure, and the
proposal that `check` should accept a `subtype?` predicate parameter
so each consumer (Go untyped-constants, TS variance, Rust lifetimes)
plugs in its own variance discipline without rewriting the judgment.

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

plans/go-on-sx.md

@@ -214,22 +214,28 @@ Progress-log line → push `origin/loops/go`.
   deferred to a follow-up; it doesn't gate Phase 3.
 
 ### Phase 3 — Bidirectional type checker, MVP (`lib/go/types.sx`) ⬜
-- **Independent implementation.** Do NOT use lib/guest/static-types-
-  bidirectional/ — that kit doesn't exist yet and depends on this work
-  for its design. See `plans/lib-guest-static-types-bidirectional.md`.
-- Synth + check judgments. Context as a value (per-block scope).
-- Coverage MVP: declared-type variables, function signatures (params +
-  returns), call type-checking, simple composite types (slice, map, chan
-  element), interface satisfaction (structural match against method sets),
-  short variable declaration `:=` (synth from RHS).
-- **Untyped constants.** `42` has type `untyped int` until contextualised;
-  this is the canonical pitfall (see Gotchas below).
-- Defer: generics (Phase 7), full conversion rules.
-- Tests: positive (type-correct programs check) + negative (mismatched
-  types fail with informative errors carrying AST paths).
-- **Acceptance:** types/ suite at 60+ tests. Chisel note `shapes-static-
-  types-bidirectional` — append a paragraph to the sister plan's design
-  diary describing what synth/check shape emerged.
+- [x] Scaffold: `go-synth` / `go-check` skeletons; context-as-value
+      (`go-ctx-empty` / `-extend` / `-lookup` / `-extend-field`);
+      predeclared `true`/`false`/`nil`; structural type equality.
+- [ ] Literal kinds in AST (parser change: `(:literal KIND VALUE)`)
+      + literal synth (`:ty-untyped-int`/`-float`/`-string`/`-rune`).
+- [ ] Binary-op synth with untyped-constant flow (canonical pitfall:
+      `var x float64 = 42 / 7` must compute as untyped int / int = 6,
+      then convert to float64).
+- [ ] Var/const declaration checking (`var x T = expr`, `var x = expr`,
+      `const Pi = 3.14`).
+- [ ] Function declaration: extend ctx with params via `:field` group,
+      check body, verify return-list types match signature.
+- [ ] Call type-checking (synth callee, check args against param types,
+      synth result).
+- [ ] Composite type element checking (slice / map / chan).
+- [ ] Interface satisfaction (structural match against method sets).
+- [ ] Short variable declaration `:=` (synth RHS into LHS bindings).
+- Defer: generics (Phase 7), full conversion rules, type assertions,
+  type switches.
+- **Acceptance:** types/ suite at 60+ tests. Current: 12/12. Chisel note
+  `shapes-static-types-bidirectional` — sister-plan design diary is the
+  cross-language record.
 
 ### Phase 4 — Tree-walk evaluator (`lib/go/eval.sx`) ⬜
 - AST-walking interpreter over CEK. Each Go statement maps to one step
@@ -528,6 +534,16 @@ Minimal repro: see `lib/go/lex.sx#gl-oct-digit?` and `#gl-match-op`.
 
 _Newest first. Append one dated entry per commit._
 
+- 2026-05-27 — **Phase 3 scaffold.** First `lib/go/types.sx` cut: context
+  as an association list (`go-ctx-empty` + `-extend` + `-lookup`), a
+  load-bearing `go-ctx-extend-field` that consumes the `:field` binding-
+  group shape (validating the Phase 2 cross-deliverable observation),
+  predeclared `true`/`false`/`nil`, `go-synth` for identifier lookup,
+  `go-check` deferring to synth + structural equality. types suite
+  12/12, total 317/317. Literal kinds (untyped int/float/string/rune)
+  + binop synth + var-decl checking next. `[shapes-static-types-
+  bidirectional]` — sister-plan diary updated with the synth/check
+  Go-side surface as it emerges.
 - 2026-05-27 — **Phase 2 complete.** End-to-end multi-form file parsing.
   `go-parse` now returns single forms for backward compat (~169 tests
   unchanged) or `(list :file FORMS)` for multi-form input. Tests cover