check_project_rules.py only checks fences + local links, so a bold marker that
CJK punctuation stops from rendering (e.g. `**（配比）**相互交织` — the closer is
preceded by punctuation and followed by a letter, so it can't close and the
asterisks print literally) would pass this repo's own CI. That class of bug had
to be swept out of every book by hand; nothing here was watching for it.
Adds check_emphasis.py — a standalone gate (identical across all books) that
runs CommonMark's flanking + process-emphasis rules and flags markers that
print literally or silently bold the wrong span. The ci.yaml content step now
runs it right after check_project_rules.py. Verified clean on this book.
