go: lex.sx — raw string literals (backtick) + 9 tests [nothing]

Adds Go raw string literals per Go spec § String literals:
backtick-delimited, no escape processing, may span multiple
lines, '\r' chars discarded from the value.

gl-read-raw-string! mirrors gl-read-string! but skips escape
handling and the \r filter. scan! routes the leading backtick
to it; emits "string" type (same as interpreted strings — no
need to distinguish at parse/type time).

lex 123/123.

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

lib/go/scoreboard.md

@@ -1,10 +1,10 @@
 # Go-on-SX Scoreboard
 
-**Total: 114 / 114 tests passing**
+**Total: 123 / 123 tests passing**
 
 |  | Suite | Pass | Total |
 |---|---|---|---|
-| ✅ | lex | 114 | 114 |
+| ✅ | lex | 123 | 123 |
 | ⬜ | parse | 0 | 0 |
 | ⬜ | types | 0 | 0 |
 | ⬜ | eval | 0 | 0 |