go: parse.sx scaffold — primary expressions + Go precedence table + 17 tests [consumes-pratt consumes-ast]

Starts Phase 2. lib/go/parse.sx defines:
  * go-precedence-table — Go's five operator-precedence levels in the
    (NAME PREC ASSOC) entry shape from lib/guest/pratt.sx, ready for the
    binary-operator iteration to consume via pratt-op-lookup.
  * go-parse(src) — tokenises and parses ONE primary expression: int,
    float, imag, string, rune literals become (ast-literal VALUE);
    identifiers become (ast-var NAME). Built directly on lib/guest/ast.sx
    constructors — no intermediate AST shape.

Conformance.sh extended to load lib/guest/{ast,pratt}.sx and run the
new parse suite. Scoreboard cleanup: drop the "pending" parse row since
the suite is now real.

parse 17/17 (lex still 129/129). Total 146/146.

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

lib/go/scoreboard.json

@@ -1,10 +1,10 @@
 {
   "language": "go",
-  "total_pass": 129,
-  "total": 129,
+  "total_pass": 146,
+  "total": 146,
   "suites": [
     {"name":"lex","pass":129,"total":129,"status":"ok"},
-    {"name":"parse","pass":0,"total":0,"status":"pending"},
+    {"name":"parse","pass":17,"total":17,"status":"ok"},
     {"name":"types","pass":0,"total":0,"status":"pending"},
     {"name":"eval","pass":0,"total":0,"status":"pending"},
     {"name":"runtime","pass":0,"total":0,"status":"pending"},