fix(compiler): skip reload command for one-shot builds

Problem: compile() unconditionally resolved the reload command, so
:Preview build on a provider with reload = function/table (e.g. typst)
would start a long-running process (typst watch) instead of a one-shot
compile (typst compile). Error diagnostics were also lost because
typst watch does not exit non-zero on input errors.

Solution: add an opts parameter to compile() with a oneshot flag.
M.build() passes { oneshot = true } so resolve_reload_cmd() is
skipped and the one-shot path is always taken. toggle() continues
to call compile() without the flag, preserving long-running behavior
for watch mode.

lua/preview/init.lua

@@ -153,7 +153,7 @@ function M.build(bufnr)
   end
   local ctx = M.build_context(bufnr)
   local provider = config.providers[name]
-  compiler.compile(bufnr, name, provider, ctx)
+  compiler.compile(bufnr, name, provider, ctx, { oneshot = true })
 end
 
 ---@param bufnr? integer