go: types.sx — literal synth + binop + assignability; canonical pitfall handled + 16 tests [shapes-static-types-bidirectional]

Phase 3 cont. Adds:

  * go-classify-literal-string — heuristic detection of literal kind
    from the value-string (parser strips lexer's kind tag; flagged for
    follow-up to extend AST shape).
  * go-synth-literal — :ty-untyped-int / -float / -imag / -string.
  * go-synth-binop — arithmetic, bitwise, comparison, logical ops with
    untyped-constant unification:
      untyped-int + untyped-float → untyped-float
      untyped + typed              → typed
      comparison ops               → bool
      logical ops                  → bool
  * go-untyped? + go-type-assignable? — pluggable assignability that
    swaps in where structural equality used to gate go-check. Untyped
    int assignable to any numeric type; untyped float assignable to
    float/complex; untyped string to string.

**Canonical Go pitfall handled correctly**: `var x float64 = 42 / 7`
parses to a binop, synth produces :ty-untyped-int (since BOTH operands
are untyped, the int division stays in the int domain), and check
against float64 returns :ok via assignability. Wrong implementations
that float-coerce eagerly would give 6.0; the right behaviour is
"compute 6 as int, then convert to float64 = 6.0".

Verified by test "binop: 42 / 7 assignable to float64 (canonical
pitfall)" and the type-only test "binop: 42 / 7 — untyped int".

Sister-plan static-types-bidirectional diary updated with the
**pluggable-assignable-predicate** kit-API proposal:

  (check-with assignable? CTX EXPR EXPECTED)

Each consumer plugs in its own variance discipline (Go untyped-flow,
TS structural subtyping, Rust lifetime-aware identity) without
rewriting synth or the judgment skeleton.

types 28/28, total 333/333.

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

lib/go/tests/types.sx

@@ -106,6 +106,91 @@
   (list :type-error :unbound "ghost"))
 
 ;; ── report ──────────────────────────────────────────────────────
+(go-types-test
+  "synth: int literal — untyped int"
+  (gtsy go-ctx-empty "42")
+  (list :ty-untyped-int))
+
+(go-types-test
+  "synth: float literal — untyped float"
+  (gtsy go-ctx-empty "3.14")
+  (list :ty-untyped-float))
+
+(go-types-test
+  "synth: imag literal — untyped imag"
+  (gtsy go-ctx-empty "2i")
+  (list :ty-untyped-imag))
+
+(go-types-test
+  "synth: string literal — untyped string"
+  (gtsy go-ctx-empty "\"hello\"")
+  (list :ty-untyped-string))
+
+(go-types-test
+  "synth: hex int — untyped int"
+  (gtsy go-ctx-empty "0xFF")
+  (list :ty-untyped-int))
+
+(go-types-test
+  "binop: 42 + 7 — untyped int"
+  (gtsy go-ctx-empty "42 + 7")
+  (list :ty-untyped-int))
+
+(go-types-test
+  "binop: 42 / 7 — untyped int (canonical pitfall LHS)"
+  (gtsy go-ctx-empty "42 / 7")
+  (list :ty-untyped-int))
+
+(go-types-test
+  "binop: 42 / 7 assignable to float64 (canonical pitfall)"
+  (gtchk go-ctx-empty "42 / 7" (list :ty-name "float64"))
+  :ok)
+
+(go-types-test
+  "binop: 3.14 * 2.0 — untyped float"
+  (gtsy go-ctx-empty "3.14 * 2.0")
+  (list :ty-untyped-float))
+
+(go-types-test
+  "binop: 1 + 2.5 — untyped int + untyped float → untyped float"
+  (gtsy go-ctx-empty "1 + 2.5")
+  (list :ty-untyped-float))
+
+(go-types-test
+  "binop: comparison produces bool"
+  (gtsy go-ctx-empty "1 < 2")
+  (list :ty-name "bool"))
+
+(go-types-test
+  "binop: typed-var + untyped-int — propagates var's type"
+  (go-synth
+    (go-ctx-extend go-ctx-empty "x" (list :ty-name "int64"))
+    (go-parse "x + 1"))
+  (list :ty-name "int64"))
+
+(go-types-test
+  "assign: untyped-int → int"
+  (gtchk go-ctx-empty "42" (list :ty-name "int"))
+  :ok)
+
+(go-types-test
+  "assign: untyped-int → float32"
+  (gtchk go-ctx-empty "42" (list :ty-name "float32"))
+  :ok)
+
+(go-types-test
+  "assign: untyped-int → string fails"
+  (gtchk go-ctx-empty "42" (list :ty-name "string"))
+  (list
+    :type-error :mismatch
+    (list :ty-name "string")
+    (list :ty-untyped-int)))
+
+(go-types-test
+  "assign: untyped-string → string"
+  (gtchk go-ctx-empty "\"hi\"" (list :ty-name "string"))
+  :ok)
+
 (define
   go-types-test-summary
   (str "types " go-types-test-pass "/" go-types-test-count))