refactor(extensions): reorganize icon management with filter-metadata

- Move icons from extension directory to organized assets location
- Create book/assets/images/icons/callouts/ for better asset management
- Use filter-metadata section for extension configuration instead of inline YAML
- Add icon-path and icon-format configuration in filter-metadata
- Update Lua filter to read from filter-metadata instead of direct YAML
- Make icon format configurable (PDF for LaTeX, PNG for HTML)
- Fix inconsistent icon filename capitalization
- Remove old icon directory from extension

This follows software engineering best practices:
- Separation of concerns (logic vs assets)
- Better maintainability and discoverability
- Consistent configuration pattern with other filters
- Future-proof asset management structure

book/_extensions/mlsysbook-ext/custom-numbered-blocks/custom-numbered-blocks.lua

@@ -704,12 +704,17 @@ insertStylesPandoc = function(doc)
   -- if stylez.extractStyleFromYaml then stylez.extractStyleFromYaml() end
   if stylez.insertPreamble and (quarto.doc.is_format("html") or quarto.doc.is_format("pdf"))
     then 
-      -- Set icon path from YAML configuration if available
+      -- Set icon path from filter-metadata configuration if available
       local meta = doc.meta
-      local cunumbl = meta["mlsysbook-ext/custom-numbered-blocks"]
-      if cunumbl and cunumbl["icon-path"] then
-        local iconPath = str(cunumbl["icon-path"])
-        quarto.doc.include_text("in-header", "\\renewcommand{\\fbxIconPath}{" .. iconPath .. "}")
+      local filterMetadata = meta["filter-metadata"]
+      if filterMetadata and filterMetadata["mlsysbook-ext/custom-numbered-blocks"] then
+        local config = filterMetadata["mlsysbook-ext/custom-numbered-blocks"]
+        if config["icon-path"] then
+          local iconPath = str(config["icon-path"])
+          local iconFormat = str(config["icon-format"] or "pdf")
+          quarto.doc.include_text("in-header", "\\renewcommand{\\fbxIconPath}{" .. iconPath .. "}")
+          quarto.doc.include_text("in-header", "\\renewcommand{\\fbxIconFormat}{" .. iconFormat .. "}")
+        end
       end
       stylez.insertPreamble(doc, fbx.classDefaults, fmt) 
     end

book/_extensions/mlsysbook-ext/custom-numbered-blocks/style/foldbox.tex

@@ -1,7 +1,8 @@
 %%%% ---foldboxy preamble ----- %%%%%
 
-% Icon path configuration - can be overridden in YAML
-\providecommand{\fbxIconPath}{_extensions/mlsysbook-ext/custom-numbered-blocks/style/icons}
+% Icon path and format configuration - can be overridden in filter-metadata
+\providecommand{\fbxIconPath}{assets/images/icons/callouts}
+\providecommand{\fbxIconFormat}{pdf}
 
 % Legacy fallback colors (keep for compatibility)
 \definecolor{fbx-default-color1}{HTML}{c7c7d0}
@@ -28,7 +29,7 @@
   overlay={
       \node [left,outer sep=0em, black,draw=none,anchor=west,
         rectangle,fill=none,inner sep=0pt]
-        at ([xshift=4mm]frame.west) {\includegraphics[width=4.2mm]{\fbxIconPath/icon_#1.pdf}};
+        at ([xshift=4mm]frame.west) {\includegraphics[width=4.2mm]{\fbxIconPath/icon_#1.\fbxIconFormat}};
     },
   },
   colframe=#1-color2,             % Border color (auto-generated from YAML)
@@ -69,7 +70,7 @@
   overlay={
       \node [left,outer sep=0em, black,draw=none,anchor=west,
         rectangle,fill=none,inner sep=0pt]
-        at ([xshift=3mm]frame.west) {\includegraphics[width=4.2mm]{\fbxIconPath/icon_#1.pdf}};
+        at ([xshift=3mm]frame.west) {\includegraphics[width=4.2mm]{\fbxIconPath/icon_#1.\fbxIconFormat}};
     },
   },
   colframe=#1-color2,             % Border color (auto-generated from YAML)

book/config/_quarto-html.yml

@@ -395,6 +395,11 @@ filter-metadata:
     file: "part_summaries.yml"
     enabled: true
 
+  # Custom numbered blocks configuration for mlsysbook-ext/custom-numbered-blocks filter
+  mlsysbook-ext/custom-numbered-blocks:
+    icon-path: "assets/images/icons/callouts"  # Path to callout icons
+    icon-format: "png"                         # Icon format for HTML builds
+
 diagram:
   engine:
     dot: true
@@ -513,7 +518,6 @@ format:
     citeproc: true
 
 mlsysbook-ext/custom-numbered-blocks:
-  icon-path: "_extensions/mlsysbook-ext/custom-numbered-blocks/style/icons"
   groups:
     quiz-question:
       colors: ["F0F0F8", "5B4B8A"]  # Darker purple for quiz questions

book/config/_quarto-pdf.yml

@@ -267,6 +267,11 @@ filter-metadata:
     file: "part_summaries.yml"              # Path to part summaries YAML file
     enabled: true                           # Enable/disable part summary injection
 
+  # Custom numbered blocks configuration for mlsysbook-ext/custom-numbered-blocks filter
+  mlsysbook-ext/custom-numbered-blocks:
+    icon-path: "assets/images/icons/callouts"  # Path to callout icons
+    icon-format: "pdf"                         # Icon format for PDF builds
+
 diagram:
   engine:
     dot: true
@@ -443,7 +448,6 @@ format:
           \let\Needspace\needspace
 
 mlsysbook-ext/custom-numbered-blocks:
-  icon-path: "_extensions/mlsysbook-ext/custom-numbered-blocks/style/icons"
   groups:
     quiz-question:
       colors: ["F0F0F8", "5B4B8A"]  # Darker purple for quiz questions