go: types.sx — composite-literal element checking; Phase 3 bar crossed + 10 tests [nothing]

Phase 3 cont. Adds composite-literal type-checking via go-synth-composite:

  []T{...}     — go-check-composite-elems with VAL-TY=T, KEY-TY=nil.
                 Each plain elem assignable to T; :kv element accepted
                 (Go's index-keyed shorthand: `[]int{0: 5, 1: 10}`)
                 with only the value checked.
  [N]T{...}    — same as slice; result :ty-array N T.
  map[K]V{...} — KEY-TY=K, VAL-TY=V. Each :kv pair: key assignable
                 to K, value to V. Non-:kv elements in maps are
                 (:type-error :map-elem-missing-key).

The literal's *synthesised* type is the type expression itself, so
nested composites fall out by recursion:

  [][]int{[]int{1,2}, []int{3,4}}
    → outer: go-check-composite-elems with VAL-TY=[]int
    → each inner []int{1,2} goes through go-synth-composite recursively,
      yielding :ty-slice :ty-name "int" — assignable-equal to VAL-TY.

Coverage: positive cases (homogeneous slices/arrays/maps, empty
slice, nested), and three negative cases (slice element mismatch,
map key mismatch, map value mismatch). Also a decl test:
  var x = []int{1, 2, 3}  →  binds x to :ty-slice :ty-name "int"

Named-type literals (`Point{1,2}`, `pkg.T{...}`) need type-decl-driven
field resolution; deferred. Interface satisfaction and AST-path error
context also remain — neither gates Phase 4.

**Phase 3 acceptance bar (60+) crossed: types 65/65, total 370/370.**

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

lib/go/tests/types.sx

@@ -415,6 +415,67 @@
     (go-parse "double(42)"))
   (list :ty-name "int"))
 
+(go-types-test
+  "composite: []int{1,2,3} — synth slice type"
+  (gtsy go-ctx-empty "[]int{1, 2, 3}")
+  (list :ty-slice (list :ty-name "int")))
+
+(go-types-test
+  "composite: []string{\"a\",\"b\"}"
+  (gtsy go-ctx-empty "[]string{\"a\", \"b\"}")
+  (list :ty-slice (list :ty-name "string")))
+
+(go-types-test
+  "composite: []int{1, \"bad\"} — element type-error"
+  (gtsy go-ctx-empty "[]int{1, \"bad\"}")
+  (list
+    :type-error :mismatch
+    (list :ty-name "int")
+    (list :ty-untyped-string)))
+
+(go-types-test
+  "composite: empty []int{}"
+  (gtsy go-ctx-empty "[]int{}")
+  (list :ty-slice (list :ty-name "int")))
+
+(go-types-test
+  "composite: [3]int{1,2,3} array"
+  (gtsy go-ctx-empty "[3]int{1, 2, 3}")
+  (list :ty-array (list :literal "3") (list :ty-name "int")))
+
+(go-types-test
+  "composite: map[string]int — synth map type"
+  (gtsy go-ctx-empty "map[string]int{\"a\": 1, \"b\": 2}")
+  (list :ty-map (list :ty-name "string") (list :ty-name "int")))
+
+(go-types-test
+  "composite: map value type-error"
+  (gtsy go-ctx-empty "map[string]int{\"a\": \"bad\"}")
+  (list
+    :type-error :mismatch
+    (list :ty-name "int")
+    (list :ty-untyped-string)))
+
+(go-types-test
+  "composite: map key type-error"
+  (gtsy go-ctx-empty "map[string]int{42: 1}")
+  (list
+    :type-error :mismatch
+    (list :ty-name "string")
+    (list :ty-untyped-int)))
+
+(go-types-test
+  "composite: nested [][]int{[]int{1,2}, []int{3,4}}"
+  (gtsy go-ctx-empty "[][]int{[]int{1, 2}, []int{3, 4}}")
+  (list :ty-slice (list :ty-slice (list :ty-name "int"))))
+
+(go-types-test
+  "composite: var x = []int{1,2,3} — inferred slice"
+  (go-ctx-lookup
+    (go-check-decl go-ctx-empty (go-parse "var x = []int{1, 2, 3}"))
+    "x")
+  (list :ty-slice (list :ty-name "int")))
+
 (define
   go-types-test-summary
   (str "types " go-types-test-pass "/" go-types-test-count))