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>

SPEC.md

@@ -1,7 +1,7 @@
 # nuwiki — Project Specification
 
-> Last updated: 2026-05-08
-> Status: In design — Phase 0 not yet started
+> Last updated: 2026-05-10
+> Status: Phase 1 (Core AST) complete — moving to Phase 2 (Syntax Plugin Interface)
 
 ---
 
@@ -586,8 +586,8 @@ These decisions are required before or during the phase indicated.
 |---|---|---|---|---|
 | ~~P1~~ | ~~**License**~~ | ~~Phase 0~~ | ✅ **Dual MIT/Apache-2.0** | |
 | ~~P2~~ | ~~**MSRV**~~ | ~~Phase 0~~ | ✅ **stable-2 (Rust 1.83)** | |
-| P3 | **String representation in AST** | Phase 1 | `String` (owned) · `Cow<'a, str>` · `Arc<str>` | `String` simplest; `Cow` enables zero-copy parsing later |
-| P4 | **Visitor pattern** | Phase 1 | Define trait now · Defer to Phase 5 | Recommend defining interface now; avoids refactor when renderer arrives |
+| ~~P3~~ | ~~**String representation in AST**~~ | ~~Phase 1~~ | ✅ **`String` (owned)** | |
+| ~~P4~~ | ~~**Visitor pattern**~~ | ~~Phase 1~~ | ✅ **Defined in Phase 1** | Open-recursion `Visitor` trait + `walk_*` helpers |
 | P5 | **Minimum Neovim version** | Phase 9 | 0.8 (`vim.lsp.start`) · 0.11 (`vim.lsp.config`) | 0.8 = broader compat; 0.11 = cleaner Lua API |
 | P6 | **Minimum Vim version** | Phase 9 | Vim 8.2 · Vim 9.0 · Vim 9.1 | LSP client plugins work best on 9.0+; recommend documenting 9.1 as minimum |
 | P7 | **Semantic token type mapping** | Phase 7 | Standard LSP types only · Custom token types | Custom types need client-side highlight group definitions; more flexible but more setup for users |