fix(compiler): respect provider open config in M.open() (#23)

* fix(reload): bind SSE server to port 0 for OS-assigned port

Problem: the SSE reload server hardcoded port 5554, causing silent
failure when that port was already in use. bind() would fail but its
return value was never checked; listen() would also error and silently
drop via the if err then return end guard. inject() still wrote the
dead EventSource URL into the HTML, so the browser would connect to
whatever was on 5554 — or nothing — and live reload would silently
stop working.

Solution: bind to port or 0 so the OS assigns a free port, then call
getsockname() after bind to capture the actual port into actual_port.
inject() reads actual_port in preference to the hardcoded constant,
and stop() resets it. PORT = 5554 is kept only as a last-resort
fallback in inject() if actual_port is unset.

* fix(compiler): resolve output into ctx before evaluating clean command

Problem: M.clean() passes the raw ctx (no output field) to the
provider's clean function. Built-in presets work around this by
recomputing the output path inline, but custom providers using
ctx.output in their clean function receive nil.

Solution: resolve output_file from provider.output before eval, extend
ctx into resolved_ctx with the output field, and use resolved_ctx when
evaluating clean and cwd — consistent with how M.compile() handles args.

lua/preview/compiler.lua

@@ -396,10 +396,16 @@ function M.clean(bufnr, name, provider, ctx)
     return
   end
 
-  local cmd = eval_list(provider.clean, ctx)
+  local output_file = ''
-  local cwd = ctx.root
+  if provider.output then
+    output_file = eval_string(provider.output, ctx)
+  end
+  local resolved_ctx = vim.tbl_extend('force', ctx, { output = output_file })
+
+  local cmd = eval_list(provider.clean, resolved_ctx)
+  local cwd = resolved_ctx.root
   if provider.cwd then
-    cwd = eval_string(provider.cwd, ctx)
+    cwd = eval_string(provider.cwd, resolved_ctx)
   end
 
   log.dbg('cleaning buffer %d with provider "%s": %s', bufnr, name, table.concat(cmd, ' '))