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>

lib/go/scoreboard.md

@@ -1,12 +1,12 @@
 # Go-on-SX Scoreboard
 
-**Total: 305 / 305 tests passing**
+**Total: 317 / 317 tests passing**
 
 |  | Suite | Pass | Total |
 |---|---|---|---|
 | ✅ | lex | 129 | 129 |
 | ✅ | parse | 176 | 176 |
-| ⬜ | types | 0 | 0 |
+| ✅ | types | 12 | 12 |
 | ⬜ | eval | 0 | 0 |
 | ⬜ | runtime | 0 | 0 |
 | ⬜ | stdlib | 0 | 0 |