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>

SPEC.md

@@ -1,7 +1,7 @@
 # nuwiki — Project Specification
 
 > Last updated: 2026-05-10
-> Status: Phase 4 (Vimwiki Parser) complete — moving to Phase 5 (Renderer)
+> Status: Phase 5 (Renderer) complete — moving to Phase 6 (LSP Foundation)
 
 ---