phase 5: renderer trait + HtmlRenderer

Writer-based `Renderer` trait per SPEC §6.8: object-safe
`fn render(&self, doc, w: &mut dyn Write) -> io::Result<()>` plus
a `render_to_string` convenience. Object safety preserves the option
of `Box<dyn Renderer>` in the LSP layer.

`HtmlRenderer` builder accepts a link-resolution callback (via
`with_link_resolver`) and an optional enclosing template (via
`with_template`). The default resolver mirrors §9: Wiki paths become
`path.html`, interwiki links land in sibling directories
(`../wiki<N>/` / `../wn-<Name>/`), diary entries under `diary/`,
file/local schemes use literal paths, anchor-only collapses to `#`.

Renderer covers every AST node:

- block: heading (centered class), paragraph, hr, blockquote,
  preformatted (`language-<lang>` class), math block (with
  `\begin{env}` when an environment is set), lists (ordered/unordered
  + every checkbox state, plus recursive sublists), definition list
  (dt/dd), table (thead/tbody driven by `is_header`, col/row span as
  CSS class hooks), comment (as HTML comment with `--` neutralised),
  error node (as `.parse-error` span)
- inline: strong/em + bold-italic, `<del>`, code (escaped),
  `<sup>`/`<sub>`, math-inline wrapped in `\(...\)`, keyword spans
  with per-keyword class, color spans, wikilink/external/raw URL
  anchors, transclusion as `<img>` with sorted attrs

HTML escaping is byte-level (`<>&"'`). Template substitution replaces
`{{content}}` first so a body containing the literal `{{title}}`
isn't double-substituted.

Tests (31 new) cover every block + inline construct, every LinkKind
through the default resolver, HTML escaping, custom resolver
override, template with/without title metadata, and an end-to-end
smoke test on a multi-construct document.

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 4 (vimwiki parser) complete. End-to-end source → AST works
-> via `VimwikiSyntax`. Renderer and editor glue still pending.
+> **Status:** Phase 5 (HTML renderer) complete. End-to-end source → AST → HTML
+> works via `VimwikiSyntax` + `HtmlRenderer`. LSP and editor glue still pending.
 
 See [`SPEC.md`](./SPEC.md) for the full project specification.
 
@@ -17,8 +17,8 @@ See [`SPEC.md`](./SPEC.md) for the full project specification.
 | 2 | Syntax Plugin Interface | ✅ done |
 | 3 | Vimwiki Lexer | ✅ done |
 | 4 | Vimwiki Parser | ✅ done |
-| 5 | Renderer | ⏳ next |
-| 6 | LSP Foundation | ⏳ |
+| 5 | Renderer | ✅ done |
+| 6 | LSP Foundation | ⏳ next |
 | 7 | Semantic Tokens | ⏳ |
 | 8 | Navigation | ⏳ |
 | 9 | Editor Glue | ⏳ |