phase 1: core AST, spans, and Visitor

Define every node type in SPEC.md §6.4 — Document, 11 block variants,
15 inline variants (including the 4 link-related ones), plus
ListItem / DefinitionItem / TableRow / TableCell, ListSymbol,
CheckboxState, Keyword, LinkTarget, LinkKind. Every node carries a
`Span` (line + column + byte offset, 0-indexed) per §6.5.

Visitor: open-recursion trait with default `visit_*` bodies that
delegate to `walk_*` free functions. Override at any granularity;
call `walk_*` from your override to keep descending. P4 resolved by
defining the trait now so Phase 5 (renderer) and Phase 7 (semantic
tokens) can plug in without refactoring traversal.

P3 resolved: AST string fields are owned `String`. Cow / Arc<str> can
be revisited if the parser grows a zero-copy mode.

Tests: visitor smoke test builds a heading + paragraph + list + table
by hand and asserts exact visit counts (4 blocks, 12 inlines, 9 text
leaves, 1 bold, 1 external link, 1 transclusion).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/nuwiki-core/src/ast/block.rs

@@ -0,0 +1,151 @@
+//! Block-level AST nodes and their supporting types.
+//!
+//! See SPEC.md §6.4.
+
+use super::inline::InlineNode;
+use super::span::Span;
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub enum BlockNode {
+    Heading(HeadingNode),
+    Paragraph(ParagraphNode),
+    HorizontalRule(HorizontalRuleNode),
+    Blockquote(BlockquoteNode),
+    Preformatted(PreformattedNode),
+    MathBlock(MathBlockNode),
+    List(ListNode),
+    DefinitionList(DefinitionListNode),
+    Table(TableNode),
+    Comment(CommentNode),
+    Error(ErrorNode),
+}
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct HeadingNode {
+    pub span: Span,
+    /// 1..=6 — invariant enforced by the parser, not the type.
+    pub level: u8,
+    pub children: Vec<InlineNode>,
+    pub centered: bool,
+}
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct ParagraphNode {
+    pub span: Span,
+    pub children: Vec<InlineNode>,
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub struct HorizontalRuleNode {
+    pub span: Span,
+}
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct BlockquoteNode {
+    pub span: Span,
+    pub children: Vec<BlockNode>,
+}
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct PreformattedNode {
+    pub span: Span,
+    pub content: String,
+    pub language: Option<String>,
+}
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct MathBlockNode {
+    pub span: Span,
+    pub content: String,
+    pub environment: Option<String>,
+}
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct ListNode {
+    pub span: Span,
+    pub ordered: bool,
+    pub symbol: ListSymbol,
+    pub items: Vec<ListItemNode>,
+}
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct ListItemNode {
+    pub span: Span,
+    pub symbol: ListSymbol,
+    pub level: usize,
+    pub checkbox: Option<CheckboxState>,
+    pub children: Vec<InlineNode>,
+    pub sublist: Option<ListNode>,
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
+pub enum ListSymbol {
+    Dash,
+    Star,
+    Hash,
+    Numeric,
+    NumericParen,
+    AlphaParen,
+    AlphaUpperParen,
+    RomanParen,
+    RomanUpperParen,
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
+pub enum CheckboxState {
+    Empty,
+    Quarter,
+    Half,
+    ThreeQuarters,
+    Done,
+    Rejected,
+}
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct DefinitionListNode {
+    pub span: Span,
+    pub items: Vec<DefinitionItemNode>,
+}
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct DefinitionItemNode {
+    pub span: Span,
+    pub term: Option<Vec<InlineNode>>,
+    pub definitions: Vec<Vec<InlineNode>>,
+}
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct TableNode {
+    pub span: Span,
+    pub rows: Vec<TableRowNode>,
+    pub has_header: bool,
+}
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct TableRowNode {
+    pub span: Span,
+    pub cells: Vec<TableCellNode>,
+    pub is_header: bool,
+}
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct TableCellNode {
+    pub span: Span,
+    pub children: Vec<InlineNode>,
+    pub col_span: bool,
+    pub row_span: bool,
+}
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct CommentNode {
+    pub span: Span,
+    pub content: String,
+}
+
+/// Resilient parse-failure node — emitted instead of aborting the document.
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct ErrorNode {
+    pub span: Span,
+    pub raw: String,
+    pub message: String,
+}