feat: Markdown emphasis inside columns blocks

Inside a columns fenced block (verbatim, so Pandoc doesn't process
Markdown), convert *italic* / **bold** / ***bold-italic*** to wikitext
emphasis. Asterisk style, single line; precedence bold-italic > bold >
italic. Raw inline HTML still works via the existing HTML-unescape path.

Docs + example updated to use ** ** rather than <b>.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

pipelines/songs/SCHEMA.md

@@ -136,12 +136,12 @@ For parallel content — e.g. an original and its translation — write a fenced
 
 ````markdown
 ```columns
-<b>原題</b>
+**原題**
 
 一行目
 二行目
 ===
-<b>Title</b>
+**Title**
 
 first line
 second line
@@ -152,7 +152,7 @@ The renderer turns this into a flexbox row of `<poem>` columns (one per `===`-de
 
 - **Line breaks and blank lines are preserved verbatim** — that's the point of using a fenced block; Pandoc passes the body through untouched, and each column is wrapped in `<poem>` so MediaWiki keeps the line breaks.
 - **N columns**: use N−1 `===` separators. Two is the common case (original / translation).
-- **Inline markup is allowed**: HTML such as `<b>…</b>` or `<br>` written inside the block is HTML-unescaped on the way out, so it renders rather than showing as literal text.
+- **Emphasis in Markdown**: write `*italic*`, `**bold**`, or `***bold-italic***` (asterisk style, single line) — these are converted to wikitext. Raw inline HTML such as `<b>…</b>` or `<br>` also still works (it's HTML-unescaped on the way out), but you shouldn't need it.
 - This is a shared transform (`lib/wiki.py`), so it works for any pipeline, not just songs. Column width/gap styling currently lives in that transform.
 
 ## Renderer behaviour