phase 17: html export commands + extended template vars

HtmlRenderer (nuwiki-core):
- `with_var(k, v)` / `with_vars(map)` add arbitrary `{{key}}`
  substitution alongside the existing `{{title}}` / `{{content}}`.
  Substitution order: content → title → user vars, so a rendered body
  that happens to contain a literal `{{title}}` isn't double-processed.

WikiConfig (nuwiki-lsp/config):
- New `html: HtmlConfig` field with per-wiki export options matching
  vimwiki's `g:vimwiki_*` keys: `html_path`, `template_path`,
  `template_default`, `template_ext`, `template_date_format`,
  `css_name`, `auto_export`, `html_filename_parameterization`,
  `exclude_files`. Defaults: `<root>/_html` and `<root>/_templates`.
- `RawWiki` accepts all of the above through `initializationOptions`
  (and `workspace/didChangeConfiguration` reload).

New `export` module (pure helpers):
- `output_path_for` — page name → on-disk HTML path. Slugifies only
  the final segment when `html_filename_parameterization = true` so
  the `diary/` subdir survives.
- `template_for` — primary/fallback path lookup tuple.
- `fallback_template` — minimal in-memory page used when neither
  template file exists.
- `format_date` — strftime subset (`%Y`/`%m`/`%d`/`%%`) so we don't
  depend on `chrono`/`time`.
- `build_toc_html` — nested `<ul class="toc">` from the doc's
  headings, with HTML escaping.
- `compute_root_path` — `../` prefix per directory depth for
  stylesheet hrefs in nested pages.
- `is_excluded` — glob matcher supporting `*`, `**`, `?`. Used by
  the `allToHtml*` walker against `exclude_files`.
- `render_page_html` — wires the above into HtmlRenderer with
  `{{date}}`, `{{root_path}}`, `{{toc}}`, `{{css}}` populated.
- `DEFAULT_CSS` — minimal stylesheet bundled for first-time exports.

New commands:
- `nuwiki.export.currentToHtml` — render the current buffer; honours
  `%nohtml` and `%template`; ensures CSS exists.
- `nuwiki.export.allToHtml` — walk the wiki root, skip pages matched
  by `exclude_files` and `%nohtml`, and only re-export pages whose
  source is newer than the existing HTML (incremental behaviour
  matching `:VimwikiAll2HTML`).
- `nuwiki.export.allToHtmlForce` — same but unconditional re-export.
- `nuwiki.export.browse` — alias of currentToHtml that returns the
  `file://` URL of the rendered page in the response so the client
  can open it.
- `nuwiki.export.rss` — write `<html_path>/rss.xml` listing the
  wiki's diary entries newest-first.
- `did_save` handler — runs the auto-export equivalent when the
  resolved wiki has `auto_export = true`.

LSP capability:
- `text_document_sync` switched from `Kind(FULL)` to `Options(...)`
  so the server can register a `save` notification (otherwise
  clients never push `did_save` events).

Tests: 33 new in `phase17_html_export.rs` covering HtmlConfig
defaults + JSON-shape config, the `format_date` subset (including
the `%%` escape and pass-through for unsupported specifiers),
root-path computation, output-path slugification rules, the glob
matcher (single-star, globstar, question-mark, non-match),
template_for primary/fallback selection, render_page_html for the
title/content/date/root_path triad and the override-by-extras flow,
and side-effecting disk paths (write_page creates parents, picks up
on-disk template, RSS is newest-first, ensure_css is idempotent).
Total 357 tests pass; fmt + clippy clean.

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

README.md

@@ -39,7 +39,7 @@ See [`SPEC.md`](./SPEC.md) for the full project specification.
 | 14 | List & table edit commands | ✅ done (focused subset; table/list-renumber/colorize deferred) |
 | 15 | Link health + TOC/index generation | ✅ done |
 | 16 | Diary | ✅ done |
-| 17 | HTML export commands | ⏳ |
+| 17 | HTML export commands | ✅ done |
 | 18 | Multi-wiki | ⏳ |
 | 19 | Editor glue v2 (`:Vimwiki*` compat, keymaps, text objects, folding) | ⏳ |