go: parse.sx — slice/array/map/chan type expressions + 11 tests; parse acceptance crossed [proposes-ast]

Adds the bulk of Go's type-expression grammar:
  []T         →  (list :ty-slice T)
  [N]T        →  (list :ty-array N T)         — N is an expr
  map[K]V     →  (list :ty-map K V)
  chan T      →  (list :ty-chan :both T)
  chan<- T    →  (list :ty-chan :send T)
  <-chan T    →  (list :ty-chan :recv T)

gp-parse-type now dispatches on the head token: *, [, map, chan, <-,
or ident; each branch recurses for nested types. Channel direction
is encoded as :both / :send / :recv (Go-specific tag).

Coverage: nested types end-to-end — []*T, [][]int, map[string][]int,
chan map[K]V, *[]int — all via the v.(T) assertion carrier.

Logged a concrete kit-gap proposal in plans/go-on-sx.md Blockers for
canonical type-node shapes. The first six (:ty-name, :ty-sel, :ty-ptr,
:ty-slice, :ty-array, :ty-map) are universal across statically-typed
guests and worth promoting on the next consumer; channel/func shapes
stay guest-specific until a second user.

Phase 2 parse acceptance bar (80+ tests) crossed: parse 81/81, total
210/210. Func / struct / interface types and full decls + stmts still
keep Phase 2 open.

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

lib/go/scoreboard.md

@@ -1,11 +1,11 @@
 # Go-on-SX Scoreboard
 
-**Total: 199 / 199 tests passing**
+**Total: 210 / 210 tests passing**
 
 |  | Suite | Pass | Total |
 |---|---|---|---|
 | ✅ | lex | 129 | 129 |
-| ✅ | parse | 70 | 70 |
+| ✅ | parse | 81 | 81 |
 | ⬜ | types | 0 | 0 |
 | ⬜ | eval | 0 | 0 |
 | ⬜ | runtime | 0 | 0 |