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/link.rs

@@ -0,0 +1,66 @@
+//! Link-related AST nodes and supporting types.
+//!
+//! See SPEC.md §6.4. All nodes here are inline (`InlineNode` variants);
+//! they live in their own module because the link model has enough surface
+//! area (kinds, anchors, interwiki indirection) to warrant separation.
+
+use std::collections::HashMap;
+
+use super::inline::InlineNode;
+use super::span::Span;
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
+pub enum LinkKind {
+    Wiki,
+    Interwiki,
+    Diary,
+    File,
+    Local,
+    Raw,
+    AnchorOnly,
+}
+
+#[derive(Debug, Clone, Default, PartialEq, Eq, Hash)]
+pub struct LinkTarget {
+    pub kind: LinkKind,
+    pub path: Option<String>,
+    pub wiki_index: Option<usize>,
+    pub wiki_name: Option<String>,
+    pub anchor: Option<String>,
+    pub is_absolute: bool,
+    pub is_directory: bool,
+}
+
+impl Default for LinkKind {
+    fn default() -> Self {
+        Self::Wiki
+    }
+}
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct WikiLinkNode {
+    pub span: Span,
+    pub target: LinkTarget,
+    pub description: Option<Vec<InlineNode>>,
+}
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct ExternalLinkNode {
+    pub span: Span,
+    pub url: String,
+    pub description: Option<Vec<InlineNode>>,
+}
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct TransclusionNode {
+    pub span: Span,
+    pub url: String,
+    pub alt: Option<String>,
+    pub attrs: HashMap<String, String>,
+}
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct RawUrlNode {
+    pub span: Span,
+    pub url: String,
+}