Fix caption label isolation: prevent flex layout from forcing 'Figure X.' onto its own line

The .poster-section > :last-child CSS rule applied display:flex to any
element that happened to be the last child of a section, including
figure/table captions. This turned <strong> tags inside captions into
block-level flex items, isolating 'Figure X.' on its own line.

Fix: add display:block override for .figure-caption/.table-caption in
poster sections, and use &nbsp; in auto-numbered caption prefixes to
prevent mid-label line breaks.

Author: jeremymanning

docs/screenshots/poster-sample.png

@@ -554,6 +554,14 @@ p img[alt] {
   font-size: 24pt;
 }
 
+/* Captions must never become flex containers (the :last-child rule above would
+   force display:flex on them when they happen to be the last element in a
+   poster-section, turning <strong> into a block-level flex item). */
+.poster-section > .figure-caption,
+.poster-section > .table-caption {
+  display: block;
+}
+
 /* SVG charts inside poster sections should not shrink with scale wrappers */
 .poster-section div[class*="scale-"] svg {
   max-width: none;

src/cdl_slides/assets/themes/cdl-poster-theme.css

@@ -202,7 +202,7 @@ def _replace_fig(m: re.Match) -> str:
         if not caption_text:
             return m.group(0)
         fig_count += 1
-        return f'<{tag} class="figure-caption"><strong>Figure {fig_count}.</strong> {caption_text}</{tag}>'
+        return f'<{tag} class="figure-caption"><strong>Figure&nbsp;{fig_count}.</strong>&nbsp;{caption_text}</{tag}>'
 
     def _replace_tbl(m: re.Match) -> str:
         nonlocal tbl_count
@@ -211,7 +211,7 @@ def _replace_tbl(m: re.Match) -> str:
         if not caption_text:
             return m.group(0)
         tbl_count += 1
-        return f'<{tag} class="table-caption"><strong>Table {tbl_count}.</strong> {caption_text}</{tag}>'
+        return f'<{tag} class="table-caption"><strong>Table&nbsp;{tbl_count}.</strong>&nbsp;{caption_text}</{tag}>'
 
     content = re.sub(r'<(div|p) class="figure-caption">(.*?)</(?:div|p)>', _replace_fig, content)
     content = re.sub(r'<(div|p) class="table-caption">(.*?)</(?:div|p)>', _replace_tbl, content)