fix(install): guard download_bin.vim against 'compatible' mode (dein build)

The dein/vim-plug build hook runs `vim -e -s -c "source scripts/download_bin.vim"`,
which starts in 'compatible' mode. There, leading-backslash line
continuations aren't recognised, so the multi-line URL string raised
`E10: \ should be followed by /, ? or &`. The install actually succeeded
(binary downloaded), but in silent Ex mode any emitted error makes Vim
exit non-zero — so dein reported a build failure. :NuwikiInstall worked
because a normal session is 'nocompatible'.

Wrap the script in the standard `let s:cpo_save = &cpo | set cpo&vim` …
`let &cpo = s:cpo_save` guard so line continuations parse regardless of
how it's invoked. Verified the exact dein command now exits 0.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

scripts/download_bin.vim

@@ -3,6 +3,14 @@
 "
 " Invoked by Dein / vim-plug build hooks:
 "   vim -e -s -c "source scripts/download_bin.vim" -c "q"
+"
+" That invocation runs in 'compatible' mode, where leading-backslash line
+" continuations aren't recognised (they'd raise E10, and any error in silent
+" Ex mode makes Vim exit non-zero — a spurious build failure). Reset
+" 'cpoptions' to the Vim default for the duration so the script parses, and
+" restore it at the end.
+let s:cpo_save = &cpo
+set cpo&vim
 
 let s:plugin_dir = fnamemodify(resolve(expand('<sfile>:p')), ':h:h')
 let s:bin_dir    = s:plugin_dir . '/bin'
@@ -89,3 +97,5 @@ else
     echohl ErrorMsg | echom 'nuwiki: install failed' | echohl None
   endif
 endif
+
+let &cpo = s:cpo_save