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>

crates/nuwiki-lsp/src/config.rs

@@ -130,6 +130,13 @@ pub struct HtmlConfig {
     /// renderer falls back to `class="color-<name>"` when a name
     /// isn't in the dict.
     pub color_dic: std::collections::HashMap<String, String>,
+    /// External wiki→HTML converter (vimwiki's `custom_wiki2html`). When
+    /// non-empty, export shells out to it instead of the built-in renderer.
+    pub custom_wiki2html: String,
+    /// Extra args appended to the `custom_wiki2html` invocation.
+    pub custom_wiki2html_args: String,
+    /// URL prefix prepended to RSS feed / diary item links (`base_url`).
+    pub base_url: String,
 }
 
 impl Default for HtmlConfig {
@@ -145,6 +152,9 @@ impl Default for HtmlConfig {
             html_filename_parameterization: false,
             exclude_files: Vec::new(),
             color_dic: std::collections::HashMap::new(),
+            custom_wiki2html: String::new(),
+            custom_wiki2html_args: String::new(),
+            base_url: String::new(),
         }
     }
 }
@@ -634,6 +644,12 @@ struct RawWiki {
     exclude_files: Option<Vec<String>>,
     #[serde(default)]
     color_dic: Option<std::collections::HashMap<String, String>>,
+    #[serde(default)]
+    custom_wiki2html: Option<String>,
+    #[serde(default)]
+    custom_wiki2html_args: Option<String>,
+    #[serde(default)]
+    base_url: Option<String>,
     // per-wiki keys mirroring vimwiki globals. All optional so
     // existing single-wiki configs migrate without touching anything.
     #[serde(default)]
@@ -720,6 +736,15 @@ impl From<RawWiki> for WikiConfig {
         if let Some(v) = r.color_dic {
             html.color_dic = v;
         }
+        if let Some(s) = r.custom_wiki2html {
+            html.custom_wiki2html = s;
+        }
+        if let Some(s) = r.custom_wiki2html_args {
+            html.custom_wiki2html_args = s;
+        }
+        if let Some(s) = r.base_url {
+            html.base_url = s;
+        }
         let d = wiki_defaults();
         WikiConfig {
             name: r.name.unwrap_or(derived_name),