Render the column layout structure on the backend, cap at two columns, add a left-margin breakout.

render() now segments the body into .colseg wrappers and flags .multicol
itself, replacing the fragile colseg injection in pagerite.js. Columns
are capped at two; shrink-wrapped figures left-align inside columns.
New {.margin} breakout (and ::: aside) drops blocks into the left gutter
on wide viewports, falling back to in-column floats.

docs/design-principles.md

@@ -19,8 +19,8 @@ Pagerite is a single-user CMS/blog. This document records the initial high-level
 
 - Content is written in **Markdown** with powerful extensions (tables, footnotes, code highlighting, etc.).
 - **Embedded HTML is passed through unfiltered**, including inline scripts and other dynamic content the author wants to post. This is safe by the single-trusted-author assumption above.
-- Renderer: **markdown-it-py** with mdit-py-plugins (footnotes, definition lists, task lists, brace-attributes, admonitions and `::: name` containers — generic `<div class="name">` wrappers (the name may be followed by brace attributes: `::: aside {.right}`), of which `::: aside` floats as a muted side box (leaning into the empty right gutter on wide single-column pages) and `::: nocols` opts its section out of column layout; tables and strikethrough from the default preset), GitHub-style alerts (`> [!NOTE]` / TIP / IMPORTANT / WARNING / CAUTION, rendered in the admonition callout styling), with `html=True` for raw passthrough, `typographer=True` for SmartyPants-style replacements in body text (curly quotes, `--` / `---` → en / em dashes, `...` → ellipsis, `(c)` → ©, etc.), and `breaks=True` so single line breaks inside paragraphs become `<br>` — including inside blockquotes, where every newline is kept and a blank `>` line starts a new paragraph. Code spans/blocks and raw HTML are left untouched. Fenced code blocks are highlighted server-side with **Pygments** (`nowrap` spans styled by `/_assets/pygments-*.css`, which maps every token class onto the `--code-*` variables; the base stylesheet defines light and dark palette sets resolved via `light-dark()`, so each theme gets the set matching its `color-scheme` and may only retint `--code-bg` to keep the well in the page's color family); a JS copy button appears on hover. Should this prove limiting, we implement our own renderer on top of html5tagger, which we already use for all HTML generation.
-- **Files are content-addressed.** Uploads (`PUT /_api/files/{filename}`) are stored by content hash — blake3, first 6 bytes hex + original extension — and served immutable from `/_f/{hash}.ext`. Absolute URLs that survive page renames and dedupe identical content; pages no longer own files. An image standing alone in its paragraph becomes a block `<figure>` — with `<figcaption>` when it has a title; images inline with text and raw `<img>` HTML stay plain inline images. Positioning is by attribute classes: `![alt](/_f/….avif "Caption"){.right}` — `{.right}`, `{.left}` float at 30% of the text column (the caption wraps within it; an explicit `width=300` makes the figure shrink-wrap the image instead), `{.wide}` goes full bleed (viewport edge to edge, or up to the docked editor; the sidebar stacks on top of it); plain attributes like `width=300` work too. The same brace syntax on a block's last line (no blank line between) applies to the whole block: a paragraph ending with `{.wide}` becomes a full-width element that breaks out of the column layout; written on the line after a block it applies to that preceding block — this is how headings, `::: containers` and code fences take classes (a wide code fence goes full bleed like a wide figure). Headings (h1/h2) clear floats, so images never overflow into the next section.
+- Renderer: **markdown-it-py** with mdit-py-plugins (footnotes, definition lists, task lists, brace-attributes, admonitions and `::: name` containers — generic `<div class="name">` wrappers (the name may be followed by brace attributes: `::: aside {.right}`), of which `::: aside` floats as a muted side box and `{.margin}` / `::: margin` marks any block a margin note — both drop into the left margin when the layout has room for it (the sticky sidebar shares that gutter and slides over them translucently, like full-bleed images), and stay in-column floats otherwise — and `::: nocols` opts its section out of column layout; tables and strikethrough from the default preset), GitHub-style alerts (`> [!NOTE]` / TIP / IMPORTANT / WARNING / CAUTION, rendered in the admonition callout styling), with `html=True` for raw passthrough, `typographer=True` for SmartyPants-style replacements in body text (curly quotes, `--` / `---` → en / em dashes, `...` → ellipsis, `(c)` → ©, etc.), and `breaks=True` so single line breaks inside paragraphs become `<br>` — including inside blockquotes, where every newline is kept and a blank `>` line starts a new paragraph. Code spans/blocks and raw HTML are left untouched. Fenced code blocks are highlighted server-side with **Pygments** (`nowrap` spans styled by `/_assets/pygments-*.css`, which maps every token class onto the `--code-*` variables; the base stylesheet defines light and dark palette sets resolved via `light-dark()`, so each theme gets the set matching its `color-scheme` and may only retint `--code-bg` to keep the well in the page's color family); a JS copy button appears on hover. Should this prove limiting, we implement our own renderer on top of html5tagger, which we already use for all HTML generation.
+- **Files are content-addressed.** Uploads (`PUT /_api/files/{filename}`) are stored by content hash — blake3, first 6 bytes hex + original extension — and served immutable from `/_f/{hash}.ext`. Absolute URLs that survive page renames and dedupe identical content; pages no longer own files. An image standing alone in its paragraph becomes a block `<figure>` — with `<figcaption>` when it has a title; images inline with text and raw `<img>` HTML stay plain inline images. Positioning is by attribute classes: `![alt](/_f/….avif "Caption"){.right}` — `{.right}`, `{.left}` float at 30% of the text column (the caption wraps within it; an explicit `width=300` makes the figure shrink-wrap the image instead), `{.margin}` drops it into the left margin like a margin note, `{.wide}` goes full bleed (viewport edge to edge, or up to the docked editor; the sidebar stacks on top of it); plain attributes like `width=300` work too. The same brace syntax on a block's last line (no blank line between) applies to the whole block: a paragraph ending with `{.wide}` becomes a full-width element that breaks out of the column layout; written on the line after a block it applies to that preceding block — this is how headings, `::: containers` and code fences take classes (a wide code fence goes full bleed like a wide figure). Headings (h1/h2) clear floats, so images never overflow into the next section.
 
 ## Page structure and navigation
 
@@ -34,7 +34,7 @@ Pagerite is a single-user CMS/blog. This document records the initial high-level
 
 ## Reading experience
 
-- The article column is sized by the **viewport, never by content**: a symmetric grid (`1fr minmax(0, 78rem) 1fr`) with flexible gutters keeps the layout stable across navigation. The sidebar occupies the left gutter, the right gutter balances it; wide screens get columns inside long articles without changing the article's width. Columns are decided client-side (pagerite.js): the body splits into segments at h1/h2 headings and `.wide` elements (full-width separators, never inside columns), and a segment gets columns only when it holds enough text — code blocks are excluded from that measure, and a `::: nocols` container opts its whole section out.
+- The article column is sized by the **viewport, never by content**: a symmetric grid (`1fr minmax(0, 78rem) 1fr`) with flexible gutters keeps the layout stable across navigation. The sidebar occupies the left gutter, the right gutter balances it; long articles (flagged `.multicol` by the backend render) lift the 78rem cap and take everything right of the left gutter, out to the viewport edge. The backend also splits the body into `.colseg` segments at h1/h2 headings, `.wide` elements and margin-breakout blocks (full-width separators or margin boxes, never inside columns), tagging segments that hold enough text with `.cols` — code blocks are excluded from that measure, and a `::: nocols` container opts its whole section out. The CSS then fits **at most two columns** of at least 30rem per segment, so a wider window widens the pair instead of adding columns. Margin boxes (`.margin`, `::: aside`) leave the column flow for the left gutter on wide viewports and stay in-column floats below that.
 - A gentle **scroll-reveal** of headings, figures and block-level elements (IntersectionObserver). It is layout-level: articles need no support for it, and `prefers-reduced-motion` disables all motion.
 
 ## Styling