phase 4: vimwiki parser + SyntaxPlugin glue

Hand-rolled recursive-descent parser over the token stream. Resilient
per SPEC §6.7: never aborts the document — anything the dispatcher
can't claim becomes BlockNode::Error and the cursor advances, and
unterminated wikilinks / transclusions / delimiters fall back to
literal text.

Block coverage: every variant from §6.4 — Heading (with inline
content between markers), Paragraph (with soft-break across single
newlines), HorizontalRule, Blockquote (both `>` and 4-space styles),
Preformatted, MathBlock, List (with checkbox + indent-driven
sublists), DefinitionList, Table (header separator promotes the
preceding row, plus colspan / rowspan cells), Comment (single +
multiline), and page-level placeholders threaded into PageMetadata.

Inline pairing: left-to-right scan with "find next matching delim",
recursing on the slice between. `_*x*_` becomes Italic(Bold(...)),
mirroring SPEC §6.7 precedence. URLs inside `[[ ]]` route to
ExternalLinkNode; everything else to WikiLinkNode with full
LinkTarget classification (Wiki / Interwiki numbered + named /
Diary / File / Local / AnchorOnly / directory / root-relative /
filesystem-absolute).

VimwikiSyntax registers as id="vimwiki", extensions=[".wiki"] and
chains lexer + parser inside SyntaxPlugin::parse.

SPEC §4 updated to record the hand-rolled choice with rationale —
span-bearing tokens fit awkwardly into winnow/nom combinator style;
resilience and recovery are explicit in code instead.

Tests (41 new parser cases) cover every AST construct, inline
precedence including nested bold/italic, every LinkKind, transclusion
alt + attrs, resilience on unterminated links, and end-to-end
SyntaxRegistry dispatch.

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

README.md

@@ -3,8 +3,8 @@
 A Vim/Neovim plugin providing full vimwiki syntax support, implemented as a
 Rust-based language server (LSP).
 
-> **Status:** Phase 3 (vimwiki lexer) complete. Parser, renderer, and editor
-> glue still pending — not yet functional end-to-end.
+> **Status:** Phase 4 (vimwiki parser) complete. End-to-end source → AST works
+> via `VimwikiSyntax`. Renderer and editor glue still pending.
 
 See [`SPEC.md`](./SPEC.md) for the full project specification.
 
@@ -16,8 +16,8 @@ See [`SPEC.md`](./SPEC.md) for the full project specification.
 | 1 | Core AST | ✅ done |
 | 2 | Syntax Plugin Interface | ✅ done |
 | 3 | Vimwiki Lexer | ✅ done |
-| 4 | Vimwiki Parser | ⏳ next |
-| 5 | Renderer | ⏳ |
+| 4 | Vimwiki Parser | ✅ done |
+| 5 | Renderer | ⏳ next |
 | 6 | LSP Foundation | ⏳ |
 | 7 | Semantic Tokens | ⏳ |
 | 8 | Navigation | ⏳ |