feat(config): custom_wiki2html external converter + base_url for RSS (P2)

Add three per-wiki HtmlConfig keys mirroring vimwiki globals:

- custom_wiki2html / custom_wiki2html_args: when set, export shells out
  to the external converter via `sh -c` with vimwiki's exact arg order
  (<cmd> <force> <syntax> <ext> <out_dir> <in_file> <css> <tpl_path>
  <tpl_default> <tpl_ext> <root_path> <args>, `-` for empty optionals)
  instead of the built-in renderer.
- base_url: when set, diary RSS item + channel links become
  <base_url><diary_rel_path>/<date>.html instead of file:// URIs.

write_page() gains a `force` flag threaded through export_current (false),
export_all (its own flag) and the did_save auto-export path (false).

Tests: write_page_invokes_custom_wiki2html_converter,
write_rss_uses_base_url_for_public_links (html_export.rs), config
round-trip + defaults (index_and_config.rs). Docs: README, doc/nuwiki.txt,
lua config comment, vimwiki-gap.md item ticked.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

development/vimwiki-gap.md

@@ -118,8 +118,22 @@ fix site.
   custom rejected glyph both lexes and round-trips. Tests:
   `listsyms::custom_rejected_glyph_is_honoured` + config round-trip in
   `index_and_config.rs`.
-- [ ] **`custom_wiki2html` (+`_args`)** and **`base_url`** — external HTML
-  converter hook + export URL prefix. _Fix:_ `config.rs` `HtmlConfig`.
+- [x] **`custom_wiki2html` (+`_args`)** and **`base_url`** — external HTML
+  converter hook + export URL prefix. _Fix:_ `HtmlConfig` (`config.rs`) gained
+  `custom_wiki2html`, `custom_wiki2html_args`, `base_url` (all default `""`,
+  round-tripped through `RawWiki`/`From`/defaults). `write_page`
+  (`commands.rs`) now takes a `force` flag and, when `custom_wiki2html` is set,
+  hands the page to `run_custom_wiki2html()` — which shells out via `sh -c`
+  with vimwiki's exact arg order (`<cmd> <force> <syntax> <ext> <out_dir>
+  <in_file> <css> <tpl_path> <tpl_default> <tpl_ext> <root_path> <args>`, `-`
+  for empty optionals) instead of the built-in renderer. `force` is threaded
+  through `export_current` (false), `export_all` (its own flag), and the
+  `did_save` auto-export path (false). `write_rss` now prefixes the channel +
+  item links with `base_url` (`<base_url><diary_rel_path>/<date>.html`) when
+  set, falling back to the entry's `file://` URI otherwise. Tests:
+  `write_page_invokes_custom_wiki2html_converter`,
+  `write_rss_uses_base_url_for_public_links` (`html_export.rs`), config
+  round-trip + defaults (`index_and_config.rs`).
 - [ ] **`map_prefix`** — `<Leader>w` hardcoded across the map layer.
   _Fix:_ `plugin/nuwiki.vim`, `lua/nuwiki/keymaps.lua`, `ftplugin/vimwiki.vim`.
 - [x] **`<M-CR>` badd-link** — was mouse-only (`<MiddleMouse>`) / command-only