Remove Rust code, delegate server to nuwiki-rs repo

Delete all Rust crates (crates/) and Cargo files. The nuwiki-ls binary
is now built in the separate nuwiki-rs repository and downloaded at
install time from the Gitea release assets, with a cargo build fallback
that clones nuwiki-rs.

Update all documentation to reflect the split repo layout.

scripts/download_bin.vim

@@ -1,5 +1,5 @@
 " scripts/download_bin.vim — install `nuwiki-ls` from a release tarball,
-" falling back to `cargo build --release` on failure.
+" falling back to `cargo build --release` from the nuwiki-rs repo.
 "
 " Invoked by Dein / vim-plug build hooks:
 "   vim -e -s -c "source scripts/download_bin.vim" -c "q"
@@ -16,6 +16,7 @@ let s:plugin_dir = fnamemodify(resolve(expand('<sfile>:p')), ':h:h')
 let s:bin_dir    = s:plugin_dir . '/bin'
 let s:suffix     = has('win32') ? '.exe' : ''
 let s:bin        = s:bin_dir . '/nuwiki-ls' . s:suffix
+let s:rs_repo    = fnamemodify(s:plugin_dir, ':h:h') . '/nuwiki-rs'
 
 call mkdir(s:bin_dir, 'p')
 
@@ -44,13 +45,21 @@ function! s:build_from_source() abort
     echohl ErrorMsg | echom 'nuwiki: cargo not found, cannot build from source' | echohl None
     return 0
   endif
+  echom 'nuwiki: cloning nuwiki-rs …'
+  if !filereadable(s:rs_repo . '/Cargo.toml')
+    call system('git clone --depth 1 https://code.gfran.co/gffranco/nuwiki-rs.git ' . shellescape(s:rs_repo))
+    if v:shell_error != 0
+      echohl ErrorMsg | echom 'nuwiki: failed to clone nuwiki-rs' | echohl None
+      return 0
+    endif
+  endif
   echom 'nuwiki: cargo build --release …'
-  call system('cargo build --release -p nuwiki-ls --manifest-path ' . shellescape(s:plugin_dir . '/Cargo.toml'))
+  call system('cargo build --release -p nuwiki-ls --manifest-path ' . shellescape(s:rs_repo . '/Cargo.toml'))
   if v:shell_error != 0
     echohl ErrorMsg | echom 'nuwiki: cargo build failed' | echohl None
     return 0
   endif
-  let l:built = s:plugin_dir . '/target/release/nuwiki-ls' . s:suffix
+  let l:built = s:rs_repo . '/target/release/nuwiki-ls' . s:suffix
   call system('cp ' . shellescape(l:built) . ' ' . shellescape(s:bin))
   call system('chmod +x ' . shellescape(s:bin))
   return executable(s:bin)
@@ -64,10 +73,7 @@ function! s:download_release() abort
   if !executable('curl') || !executable('tar')
     return 0
   endif
-  " Gitea serves the most recent release under the `latest` tag segment
-  " (/releases/download/latest/<asset>). NOT GitHub's
-  " /releases/latest/download/<asset> shape, which Gitea 404s.
-  let l:url = 'https://code.gfran.co/gffranco/nuwiki/releases/download/latest/nuwiki-ls-'
+  let l:url = 'https://code.gfran.co/gffranco/nuwiki-rs/releases/download/latest/nuwiki-ls-'
         \ . l:target . '.tar.gz'
   let l:archive = tempname() . '.tar.gz'
   call system('curl -fsSL -o ' . shellescape(l:archive) . ' ' . shellescape(l:url))
@@ -85,11 +91,7 @@ function! s:download_release() abort
   return executable(s:bin)
 endfunction
 
-if get(g:, 'nuwiki_build_from_source', 0)
-  if !s:build_from_source()
-    echohl ErrorMsg | echom 'nuwiki: source build failed' | echohl None
-  endif
-elseif s:download_release()
+if s:download_release()
   echom 'nuwiki: installed ' . s:bin
 else
   echohl WarningMsg | echom 'nuwiki: download failed, falling back to source build' | echohl None