Keep margin boxes in the column flow, position them out of flow

::: aside / .margin blocks no longer split the column segments: they
stay inside the .colseg at their anchor point, so the surrounding text
is measured and laid out as a single columned layout (split-off sides
previously lost .cols when too short on their own).

The zone placements (multicol side zone, sidebar track, wide gutter)
now position the boxes absolutely off the article's left border —
unaffected by any column layout inside — with the vertical spot coming
from the unset top (where the box occurs in the text). Narrow widths
keep the in-column float fallback. Trade-off: out-of-flow boxes no
longer stack via clear, so boxes anchored close together may overlap.

pagerite/markdown.py

@@ -25,14 +25,18 @@ schemes stay visible; manually labelled links are untouched), and
 ``H~2~O`` / ``x^2^`` give sub/superscripts.
 
 render() also builds the layout structure: the top-level blocks are
-segmented for the column layout — h1/h2 headings, ``.wide`` blocks and
-margin-breakout blocks (``.margin``, ``::: aside``) stand on their own,
-the runs between them are wrapped in ``<div class="colseg">`` (tagged
+segmented for the column layout — h1/h2 headings and ``.wide`` blocks
+stand on their own, the runs between them are wrapped in
+``<div class="colseg">`` (tagged
 ``.cols`` when the segment holds enough text — COLS_TEXT — in at least
 COLS_PARAS paragraphs or one paragraph long enough to turn .breakable,
 unless a ``::: nocols`` container opts it out;
 in column segments, paragraphs past BREAKABLE_TEXT are marked
-``.breakable`` so they may split across columns). The result carries
+``.breakable`` so they may split across columns). Margin-breakout boxes
+(``.margin``, ``::: aside``) stay inside the segment at their anchor
+point; pagerite.css takes them out of flow (absolute, off the article's
+left border, into the side zone), so the columns flow through as if the
+box wasn't there. The result carries
 ``multicol`` when the whole body justifies columns (views.py puts the
 class on the article); how many columns (never more than two), whether
 the margin breakout applies and every other viewport adaptation is then
@@ -268,8 +272,8 @@ def _container_attrs(state) -> None:
     The container plugin's default render is a plain renderToken, so the
     name and brace attributes must live on the token itself — and being a
     core rule (rather than a render rule) lets the segmentation in
-    render() see the classes (::: aside's margin breakout, the ::: nocols
-    opt-out, {.wide} containers).
+    render() see the classes (the ::: nocols opt-out, {.wide}
+    containers).
     """
     for token in state.tokens:
         if token.type != "container_block_open":
@@ -493,11 +497,12 @@ _PARA_OPEN_RE = re.compile(r"<p[\s>]")
 _PARA_RE = re.compile(r"<p((?:\s[^>]*)?)>(.*?)</p>", re.S)
 
 # Classes that take their block out of the column flow: .wide is a
-# full-width separator, .margin/.aside float in the side zone at the
-# article's left (they must be direct article children for that — the zone
-# rules key off it — never inside a column).
+# full-width separator that splits the column segments. Margin-breakout
+# boxes (.margin/.aside) are NOT boundaries: they stay inside the segment
+# at their anchor point, and CSS positions them absolutely out of the
+# article's left border (the zone rules anchor off the article), so the
+# column flow is unaffected.
 _WIDE = "wide"
-_BREAKOUT = ("margin", "aside")
 
 
 class Rendered(NamedTuple):
@@ -557,14 +562,10 @@ def _top_level_blocks(tokens: list) -> list[list]:
 
 def _is_boundary(block: list) -> bool:
     """True for blocks that never go inside a column segment (see the
-    _WIDE/_BREAKOUT comment above): h1/h2 headings, anything carrying
-    .wide, and blocks whose own element carries .margin/.aside — for a
-    lone-image paragraph (which renders as a <figure>) the image's classes
-    count as the block's own."""
+    _WIDE comment above): h1/h2 headings and anything carrying .wide."""
     first = block[0]
     if first.type == "heading_open" and first.tag in ("h1", "h2"):
         return True
-    own = _classes(first)
     for token in block:
         if _WIDE in _classes(token):
             return True
@@ -572,9 +573,7 @@ def _is_boundary(block: list) -> bool:
             children = token.children or []
             if any(_WIDE in _classes(c) for c in children):
                 return True
-            if len(children) == 1 and children[0].type == "image":
-                own |= _classes(children[0])
-    return bool(own & set(_BREAKOUT))
+    return False
 
 
 def render(
@@ -591,8 +590,8 @@ def render(
     same pipeline as an explicit one (first-h1 anchor treatment included).
 
     The top-level blocks are grouped into column segments: boundary blocks
-    (h1/h2 headings, .wide, margin-breakout blocks — see _is_boundary) are
-    rendered bare, the runs between them wrapped in <div class="colseg">.
+    (h1/h2 headings, .wide — see _is_boundary) are rendered bare, the runs
+    between them wrapped in <div class="colseg">.
     A segment is tagged .cols when it holds enough text (COLS_TEXT) in at
     least two paragraphs (COLS_PARAS) or one breakable-length paragraph,
     and no ::: nocols container; its long paragraphs are marked .breakable;