Fix spec serialization divergences and add conformance tests

CHANGELOG.md

@@ -7,6 +7,18 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Changed
+
+#### Spec Serialization Compliance
+- **Breaking:** `Mark::Math { value }` field renamed to `Mark::Math { source }` to match spec
+- **Breaking:** Simple marks (Bold, Italic, etc.) now serialize as strings (`"bold"`) instead of objects (`{"type":"bold"}`)
+- **Breaking:** Extension marks serialize with colon-delimited type (`"semantic:citation"`) instead of wrapper (`{"type":"extension","namespace":"semantic","markType":"citation"}`)
+- **Breaking:** Extension blocks serialize with colon-delimited type (`"academic:theorem"`) instead of wrapper format
+- **Breaking:** `Block::block_type()` returns `Cow<'_, str>` instead of `&'static str`; extension blocks return `"namespace:blockType"` instead of `"extension"`
+- `FigCaption` block type serializes as `"figcaption"` (lowercase) instead of `"figCaption"`
+- All old formats are accepted on deserialization for backward compatibility
+- Added conformance test suite (`tests/conformance.rs`) to prevent future spec drift
+
 ## [0.4.0] - 2026-02-16
 
 ### Added

cdx-cli/src/commands/inspect.rs

@@ -126,7 +126,7 @@ fn display_text(
             println!(
                 "  {}. {}",
                 i + 1,
-                format_block_type(block.block_type()).cyan(),
+                format_block_type(&block.block_type()).cyan(),
             );
         }
     }
@@ -188,7 +188,7 @@ fn format_block_type(block_type: &str) -> String {
         "svg" => "SVG",
         "barcode" => "Barcode",
         "figure" => "Figure",
-        "figCaption" => "Figure Caption",
+        "figcaption" => "Figure Caption",
         other => other,
     }
     .to_string()