Make section header optional, use key as fallback

When header field is omitted, the section key is used as the header text.
This reduces verbosity for common cases:

  @ sections.Introduction { body = ... }
  @ sections."*euremark* について" { ... }  // with markdown formatting

Explicit header still supported when key differs from display text:

  @ sections.syntax { header = markdown`Syntax Overview` }

Author: claude

assets/examples/example.eumd

@@ -45,17 +45,16 @@ cites = ```bibtex
   content = markdown`See the [CommonMark Spec](https://commonmark.org) for details.`
 }
 
-@ sections.intro {
-  header = markdown`Introduction`
+// Section key as header (no explicit header field needed)
+@ sections.Introduction {
   body = ```markdown
   Traditional Markdown documents have **implicit structure** based on heading levels.
   This can lead to ambiguity and makes programmatic manipulation difficult.
 
   Eure-mark addresses this by making the section hierarchy **explicit**!footnote[impl-detail].
   ```
 
-  @ sections.motivation {
-    header = markdown`Motivation`
+  @ sections.Motivation {
     body = ```markdown
     The key motivations for eure-mark are:
 
@@ -66,8 +65,8 @@ cites = ```bibtex
     ```
   }
 
-  @ sections.goals {
-    header = markdown`Design Goals`
+  // Quoted key for markdown formatting in header
+  @ sections."Design *Goals*" {
     body = ```markdown
     1. Maintain Markdown familiarity for content
     2. Add structure where Markdown is weak
@@ -77,14 +76,14 @@ cites = ```bibtex
   }
 }
 
+// Explicit header when key differs from display text
 @ sections.syntax {
   header = markdown`Syntax Overview`
   body = ```markdown
   Eure-mark uses Eure syntax for structure and Markdown for content.
   ```
 
-  @ sections.inline {
-    header = markdown`Inline Markdown`
+  @ sections."Inline Markdown" {
     body = ````markdown
     For short text like headers, use inline code with `markdown` tag:
 
@@ -96,8 +95,7 @@ cites = ```bibtex
     ````
   }
 
-  @ sections.block {
-    header = markdown`Block Markdown`
+  @ sections."Block Markdown" {
     body = ````markdown
     For longer content, use fenced code blocks:
 
@@ -113,8 +111,7 @@ cites = ```bibtex
   }
 }
 
-@ sections.references {
-  header = markdown`References`
+@ sections.References {
   body = ```markdown
   This format was inspired by literate programming!cite[knuth1984] and
   the LaTeX document preparation system!cite[lamport1994].

assets/schemas/eure-md.schema.eure

@@ -9,9 +9,14 @@
 /// - Integrated footnotes and citations (BibTeX)
 /// - Markdown content in code blocks
 ///
-/// Section references:
-/// - Sections use map structure with ID as key: @ sections.intro { ... }
-/// - Reference in markdown: !ref[intro]
+/// Section keys as headers:
+/// - Section key is used as header if `header` field is omitted
+/// - @ sections.Introduction { body = ... }  → header is "Introduction"
+/// - @ sections."*euremark* について" { ... } → header with markdown formatting
+/// - @ sections.intro { header = markdown`Custom Header` } → explicit header
+///
+/// References:
+/// - Reference sections in markdown: !ref[intro]
 /// - Nested sections: @ sections.intro.sections.subsection { ... }
 
 $schema = "eure-schema.schema.eure"
@@ -123,9 +128,11 @@ sections.$binding-style = "section"
 }
 
 /// Section structure (recursive, map-based for referencing)
+/// If header is omitted, the section key is used as the header text.
 @ $types.section {
-  // Section header (inline markdown)
+  // Section header (inline markdown). If omitted, section key is used as header.
   header = `$types.inline-markdown`
+  header.$optional = true
 
   // Section body content (block markdown)
   body = `$types.block-markdown`