fix(presets): add --failure-level ERROR to asciidoctor args

Problem: asciidoctor exits 0 even when it encounters ERROR-level
messages, so the compiler treats every run as a success and never
invokes error_parser. Errors are silently embedded as visible text
in the HTML output instead of surfacing as diagnostics.

Solution: pass --failure-level ERROR so asciidoctor exits non-zero
on errors, which triggers error_parser and surfaces diagnostics.

lua/preview/presets.lua

@@ -246,7 +246,7 @@ M.asciidoctor = {
   ft = 'asciidoc',
   cmd = { 'asciidoctor' },
   args = function(ctx)
-    return { ctx.file, '-o', ctx.output }
+    return { '--failure-level', 'ERROR', ctx.file, '-o', ctx.output }
   end,
   output = function(ctx)
     return (ctx.file:gsub('%.adoc$', '.html'))