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/compiler.lua

@@ -55,7 +55,9 @@ end
 ---@param name string
 ---@param provider preview.ProviderConfig
 ---@param ctx preview.Context
-function M.compile(bufnr, name, provider, ctx)
+function M.compile(bufnr, name, provider, ctx, opts)
+  opts = opts or {}
+
   if vim.bo[bufnr].modified then
     vim.cmd('silent! update')
   end
@@ -81,7 +83,10 @@ function M.compile(bufnr, name, provider, ctx)
     last_output[bufnr] = output_file
   end
 
-  local reload_cmd = resolve_reload_cmd(provider, resolved_ctx)
+  local reload_cmd
+  if not opts.oneshot then
+    reload_cmd = resolve_reload_cmd(provider, resolved_ctx)
+  end
 
   if reload_cmd then
     log.dbg(