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 cwd = ctx.root
+  local output_file = ''
+  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, ' '))

lua/preview/reload.lua

@@ -2,6 +2,7 @@ local M = {}
 
 local PORT = 5554
 local server_handle = nil
+local actual_port = nil
 local clients = {}
 
 local function make_script(port)
@@ -14,12 +15,15 @@ local function make_script(port)
 end
 
 function M.start(port)
-  port = port or PORT
   if server_handle then
     return
   end
   local server = vim.uv.new_tcp()
-  server:bind('127.0.0.1', port)
+  server:bind('127.0.0.1', port or 0)
+  local sockname = server:getsockname()
+  if sockname then
+    actual_port = sockname.port
+  end
   server:listen(128, function(err)
     if err then
       return
@@ -66,6 +70,7 @@ function M.stop()
     server_handle:close()
     server_handle = nil
   end
+  actual_port = nil
 end
 
 function M.broadcast()
@@ -85,7 +90,7 @@ function M.broadcast()
 end
 
 function M.inject(path, port)
-  port = port or PORT
+  port = actual_port or port or PORT
   local f = io.open(path, 'r')
   if not f then
     return