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/scoreboard.json

@@ -1,11 +1,11 @@
 {
   "language": "go",
-  "total_pass": 360,
-  "total": 360,
+  "total_pass": 370,
+  "total": 370,
   "suites": [
     {"name":"lex","pass":129,"total":129,"status":"ok"},
     {"name":"parse","pass":176,"total":176,"status":"ok"},
-    {"name":"types","pass":55,"total":55,"status":"ok"},
+    {"name":"types","pass":65,"total":65,"status":"ok"},
     {"name":"eval","pass":0,"total":0,"status":"pending"},
     {"name":"runtime","pass":0,"total":0,"status":"pending"},
     {"name":"stdlib","pass":0,"total":0,"status":"pending"},