test(render-eval): add visible_open_double_backtick audit rule

Catches asymmetric typographic-quote emission where mandoc emits
literal \`\` for the open side while converting '' to U+201D.

Author: idank

tests/evals/render/render_eval.py

@@ -903,6 +903,7 @@ def diff_report(args: argparse.Namespace) -> int:
 _AUDIT_VISIBLE_ZWNJ_RE = re.compile(r"‌")
 _AUDIT_VISIBLE_NBSP_RE = re.compile(r" ")
 _AUDIT_VISIBLE_DOUBLE_AMP_RE = re.compile(r"&")
+_AUDIT_OPEN_DOUBLE_BACKTICK_RE = re.compile(r"\\`\\`")
 _AUDIT_SYNOPSIS_HEADING_RE = re.compile(r"^#+\s+SYNOPSIS\s*$", re.IGNORECASE)
 _AUDIT_WHITESPACE_RE = re.compile(r"\s")
 _AUDIT_UNICODE_ESCAPE_RE = re.compile(r"^u[0-9A-Fa-f]{4,6}$")
@@ -1198,6 +1199,14 @@ def _audit_scan_visible_double_amp(markdown: str, html: str) -> list[tuple[int,
     ]
 
 
+def _audit_scan_open_double_backtick(markdown: str, html: str) -> list[tuple[int, str]]:
+    hits: list[tuple[int, str]] = []
+    for line_no, line in enumerate(markdown.splitlines(), 1):
+        for match in _AUDIT_OPEN_DOUBLE_BACKTICK_RE.finditer(line):
+            hits.append((line_no, _audit_snippet(line, match.start(), match.end())))
+    return hits
+
+
 def _audit_scan_giant_markdown_line(markdown: str, html: str) -> list[tuple[int, str]]:
     hits: list[tuple[int, str]] = []
     for line_no, line in enumerate(markdown.splitlines(), 1):
@@ -1262,6 +1271,11 @@ def _audit_scan_synopsis_no_spaces_run(
         _audit_scan_visible_double_amp,
         "double-encoded ampersand &",
     ),
+    (
+        "visible_open_double_backtick",
+        _audit_scan_open_double_backtick,
+        r"literal `\`\`` in markdown (asymmetric typographic open quote)",
+    ),
     (
         "giant_markdown_line",
         _audit_scan_giant_markdown_line,