config: fail fast on missing TempCtrl PID gains

Config access was unchecked dict[key] everywhere, so a missing/
misnamed gain (we've hit this once already: config.json.sim's gain/
Model nesting mismatch) surfaced as a bare KeyError from deep inside
some component's __init__ or first process() tick, with no indication
which config key was wrong.

utils/config_validate.py adds a small stdlib-only dotted-path checker
(no schema library - matches this project's stdlib-first leaning) and
validate_temp_ctrl(), scoped to the section that actually caused an
incident: TempCtrl.pid_type plus every kp/ki/kd/kt gain under
TempCtrl.Outer and TempCtrl.Inner.{Heat,Hold,Cool}. Called once in
server/brewpi.py right after json.load(), before any factory runs.
Optional-with-default keys (Outer.y_hold_min, Inner.Hold.yi_max,
Thresholds) stay optional here too.

Heater/Stirrer/TempSensor/Pot sections and unknown-key warnings are
deliberately left for later - see components/pid/TODO.md.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01F4oEE99rkKVU8L8tkzXWdG

server/brewpi.py

@@ -10,6 +10,7 @@ import threading
 import time
 from http.server import SimpleHTTPRequestHandler, ThreadingHTTPServer
 from utils import ChangedFloat
+from utils.config_validate import validate_temp_ctrl
 from ws.message import MessageDispatcher
 from ws.server.ws_server_multi_user import WsServerMultiUser
 from components.pid import PidFactory
@@ -103,6 +104,7 @@ if __name__ == '__main__':
 	log.info("Logging to {}".format(log_path))
 
 	config = json.load(open(args.config))
+	validate_temp_ctrl(config)
 	dispatcher = MessageDispatcher()
 	server = WsServerMultiUser(listener=dispatcher)
 	taskmgr = TaskManager()