Make callout headings explicit

CONTENT_BRIDGE.md

@@ -94,7 +94,9 @@ forward, use normal Markdown and let Astro CSS handle presentation.
 
 ### Callouts
 
-Write callouts as portable blockquotes. This is the canonical form:
+Write callouts as portable blockquotes. The type controls the visual styling,
+but it does not create a visible heading by itself. This is the canonical form
+for a titleless note:
 
 ```markdown
 > **Note**
@@ -110,13 +112,31 @@ Supported labels are `Note`, `Tip`, `Warning`, `Caution`, and `Aside`:
 > This point needs attention.
 ```
 
-Obsidian-style labels also work:
+Obsidian-style bracket labels also work. A plain bracket label is still
+titleless:
 
 ```markdown
 > [!tip]
 > This is a tip.
 ```
 
+To show a heading, put it explicitly inside the bracket after `:` or `|`:
+
+```markdown
+> [!tip: Useful observation]
+>
+> Text of the callout.
+
+> [!warning|Check this assumption]
+>
+> This point needs attention.
+```
+
+The text after `:` or `|` is treated as the only callout heading. Do not write
+`> [!tip] Useful observation` for a heading; that form is interpreted as a
+titleless tip whose body begins with “Useful observation”. This keeps the
+Markdown unambiguous across Astro, Pandoc, and LaTeX.
+
 For quick writing, a few leading emoji markers are recognized and stripped:
 
 ```markdown
@@ -139,9 +159,9 @@ aside   -> blogasidebox
 
 The LaTeX boxes use `tcolorbox` with the `breakable` and `enhanced` libraries,
 a light tinted background, a thin frame, a stronger left rule, and a small
-attached title label. This follows the same design vocabulary as the DM book
-callouts: compact colored boxes that can split across pages without becoming
-visually heavy.
+attached title only when the Markdown explicitly provides one. This follows the
+same design vocabulary as the DM book callouts: compact colored boxes that can
+split across pages without becoming visually heavy.
 
 Quarto margin notes such as `[text]{.aside}` were converted to: