feat: side-by-side columns shorthand (```columns fence)

Authors write a ```columns fenced block (columns separated by a line of
===); Pandoc passes the body through verbatim as <pre class="columns">,
and a new post-Pandoc transform in lib.wiki.expand_columns expands it
into a flex <div> of <poem> columns. Runs entirely Pi-side before the
MediaWiki API write — no wiki template or PHP extension required.

The transform lives in shared lib/wiki.py (called from markdown_to_wikitext),
so it is universal across all pipelines. Stdlib only; no new deps.

Docs: SCHEMA.md author contract + songs/root decision logs.
Also ignore __pycache__/.

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

pipelines/songs/SCHEMA.md

@@ -122,20 +122,46 @@ The siblings list is a bill of materials — declaring a sibling has no effect u
 
 ## Body
 
-Plain markdown. The renderer applies two transformations before and after Pandoc:
+Plain markdown. The renderer applies these transformations before and after Pandoc:
 
 1. **Pre-Pandoc**: strip the first-line `# Heading` if present.
-2. **Post-Pandoc**: substitute `TAG` placeholders declared in `wiki.siblings`.
+2. **Post-Pandoc**: expand `columns` fenced blocks into side-by-side wikitext (see below).
+3. **Post-Pandoc**: substitute `TAG` placeholders declared in `wiki.siblings`.
 
 Do not embed raw wikitext-specific syntax (`{{Template}}`, raw `[[Wikilink]]` not declared via `siblings`, etc.) in the body unless you intend the literal output. Use markdown idioms; the renderer adds the metadata-derived bits (banner, LRC link, categories, sibling resolution) around Pandoc's output.
 
+### Columns (side-by-side) shorthand
+
+For parallel content — e.g. an original and its translation — write a fenced code block tagged `columns` and separate the columns with a line containing only `===`:
+
+````markdown
+```columns
+<b>原題</b>
+
+一行目
+二行目
+===
+<b>Title</b>
+
+first line
+second line
+```
+````
+
+The renderer turns this into a flexbox row of `<poem>` columns (one per `===`-delimited section). Notes:
+
+- **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.
+- 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
 
 For each `.md` with `wiki.publish: true` and not under `wip/`:
 
 1. Parse and validate frontmatter.
 2. Strip the leading `# Heading` from the body if present.
-3. Pipe the body through `pandoc -f markdown -t mediawiki`.
+3. Pipe the body through `pandoc -f markdown -t mediawiki`, then expand any `columns` fenced blocks into side-by-side `<poem>` columns.
 4. Substitute `TAG` placeholders with resolved `[[Page|Display]]` wikilinks (or fallback text).
 5. Prepend the auto-generated banner: `{{Auto-generated|source=<source URL>|commit=<sha>}}`.
 6. If `lrc` is declared: upload the LRC file if its SHA1 has changed; append a `[[Media:…]]` link.