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.md

@@ -1,11 +1,11 @@
 # Go-on-SX Scoreboard
 
-**Total: 129 / 129 tests passing**
+**Total: 146 / 146 tests passing**
 
 |  | Suite | Pass | Total |
 |---|---|---|---|
 | ✅ | lex | 129 | 129 |
-| ⬜ | parse | 0 | 0 |
+| ✅ | parse | 17 | 17 |
 | ⬜ | types | 0 | 0 |
 | ⬜ | eval | 0 | 0 |
 | ⬜ | runtime | 0 | 0 |