go: eval.sx — structs + selector + selector-assign + 8 tests [nothing]

Phase 4 cont. Adds runtime support for Go's struct type.

Struct representation: (list :go-struct TYPE-NAME FIELDS) where
FIELDS is an association list of (field-name value) pairs.

`type T struct { ... }` is now significant at eval-time. The new
go-eval-type-decl registers field-name lists in env under
(:go-struct-type FIELD-NAMES) so positional composite literals can
map argument positions to field names. Non-struct type aliases are
silent no-ops in v0.

go-eval-composite extended:
  * If type is (:var TYPE-NAME), look up in env. Must be a
    :go-struct-type entry — error otherwise.
  * go-eval-struct-lit branches on whether the first elem is :kv
    (keyed) or not (positional). Keyed mode reads key-name from each
    :kv's key (which is a :var node). Positional mode arity-checks
    against the field-names list and zips positionally.

go-eval-select handles (:select OBJ FIELD-NAME) — field lookup with
go-map-get on the FIELDS assoc list.

go-eval-assign-pairs gets a new (:select OBJ FIELD) LHS branch:
  - var-rooted only for v0
  - rebuilds the struct via go-map-set, rebinds the var

**Functions taking and returning structs round-trip end-to-end:**

  type Point struct { x, y int }
  func add(a, b Point) Point { return Point{a.x + b.x, a.y + b.y} }
  add(Point{1, 2}, Point{3, 4})  // Point{4, 6}

Method-dispatch (calling p.M() where M is a method on Point's type)
is the next step; needs threading the type checker's #method/T/N
scheme into eval-time so functions can be looked up by receiver type.

eval 66/66, total 443/443.

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

lib/go/scoreboard.json

@@ -1,12 +1,12 @@
 {
   "language": "go",
-  "total_pass": 435,
-  "total": 435,
+  "total_pass": 443,
+  "total": 443,
   "suites": [
     {"name":"lex","pass":129,"total":129,"status":"ok"},
     {"name":"parse","pass":176,"total":176,"status":"ok"},
     {"name":"types","pass":72,"total":72,"status":"ok"},
-    {"name":"eval","pass":58,"total":58,"status":"ok"},
+    {"name":"eval","pass":66,"total":66,"status":"ok"},
     {"name":"runtime","pass":0,"total":0,"status":"pending"},
     {"name":"stdlib","pass":0,"total":0,"status":"pending"},
     {"name":"e2e","pass":0,"total":0,"status":"pending"}