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/lib.rs

@@ -2,3 +2,5 @@
 //!
 //! This crate is editor-independent and must never depend on `nuwiki-lsp` or
 //! `nuwiki-ls`. See SPEC.md §6.2 for the dependency rules.
+
+pub mod ast;