feat(diary): -count wiki selector for diary-note + index commands (P3)

Closes the last command-cluster gap. Upstream's diary-note family
(:Vimwiki{Make,TabMake,MakeYesterday,MakeTomorrow}DiaryNote + VimwikiDiaryIndex,
and the Nuwiki* forms) carry -count=0 where the count selects the wiki number;
nuwiki's were bare. Implemented as a real end-to-end selector, reusing existing
infrastructure:

- Server (commands.rs): OptUriArg gained an optional `wiki` selector;
  resolve_diary_wiki(backend, uri, wiki) now prefers it (via the existing
  resolve_wiki_selector, which already handles a 0-indexed wiki number) over
  the buffer URI. Used by diary_open_relative + diary_open_index.

- Clients: s:diary_open / _diary_open (both) and diary_today/today_tab/
  yesterday/tomorrow/index take a count and send {wiki: count-1} when >0. All
  22 diary-note + DiaryIndex command defs across the 4 contexts are now
  -count=0 passing <count>. diary_next/prev ignore the count.

Tests: cmd.VimwikiMakeDiaryNote_has_count (test-keymaps.lua) +
cmd.Vimwiki{MakeDiaryNote,DiaryIndex}_accepts_count (test-keymaps-vim.vim).
Workspace test/clippy/fmt clean with CI flags; lua 284, vim 272/18/21.

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

crates/nuwiki-lsp/src/commands.rs

@@ -392,7 +392,15 @@ enum RelativeDay {
     Tomorrow,
 }
 
-fn resolve_diary_wiki(backend: &Backend, uri: Option<&Url>) -> Option<crate::wiki::Wiki> {
+fn resolve_diary_wiki(
+    backend: &Backend,
+    uri: Option<&Url>,
+    wiki: Option<&Value>,
+) -> Option<crate::wiki::Wiki> {
+    // An explicit wiki selector (vimwiki's `-count`) wins over the buffer URI.
+    if let Some(sel) = wiki {
+        return resolve_wiki_selector(backend, Some(sel));
+    }
     if let Some(u) = uri {
         if let Some(w) = backend.wiki_for_uri(u) {
             return Some(w);
@@ -407,7 +415,7 @@ fn diary_open_relative(
     rel: RelativeDay,
 ) -> Result<Option<Value>, String> {
     let p = parse_optional_uri_arg(args)?;
-    let Some(wiki) = resolve_diary_wiki(backend, p.uri.as_ref()) else {
+    let Some(wiki) = resolve_diary_wiki(backend, p.uri.as_ref(), p.wiki.as_ref()) else {
         return Ok(None);
     };
     let freq = wiki.config.frequency();
@@ -430,7 +438,7 @@ fn diary_open_relative(
 
 fn diary_open_index(backend: &Backend, args: Vec<Value>) -> Result<Option<Value>, String> {
     let p = parse_optional_uri_arg(args)?;
-    let Some(wiki) = resolve_diary_wiki(backend, p.uri.as_ref()) else {
+    let Some(wiki) = resolve_diary_wiki(backend, p.uri.as_ref(), p.wiki.as_ref()) else {
         return Ok(None);
     };
     let Some(uri) = crate::diary::index_uri(&wiki.config) else {
@@ -444,7 +452,7 @@ fn diary_generate_index(
     args: Vec<Value>,
 ) -> Result<Option<WorkspaceEdit>, String> {
     let p = parse_optional_uri_arg(args)?;
-    let Some(wiki) = resolve_diary_wiki(backend, p.uri.as_ref()) else {
+    let Some(wiki) = resolve_diary_wiki(backend, p.uri.as_ref(), p.wiki.as_ref()) else {
         return Ok(None);
     };
     let utf8 = backend.use_utf8.load(Ordering::Relaxed);