feat: add Dreo 712S support, light platform, protocol extensions, example config - #2

The 712S (ESP32-C3) is a fan + main CCT light + locally-driven ambient RGB
unit. Its module is already an ESP32-C3, so unlike the HTF018S it needs no
hardware swap - just a stock-compatible partition table.

Component changes:

- New light platform (components/dreo/light) for datapoint-backed lights:
  switch/brightness/color_temperature datapoints, configurable mired range,
  and the stock "higher = cooler" colour-temp inversion. Guards against
  echoing MCU-sourced updates back to the MCU.
- Protocol: parse string datapoints (type 0x03), accept 1/2/4-byte integers
  rather than 4-byte only, and treat 0x0E as the remote/panel button event it
  actually is (previously ignored as undecipherable). Adds the 0x09 version
  query and 0x03 wifi-status frames some models expect during sync.
- New hub options, all defaulting to existing behaviour: startup_heartbeat_
  interval, max_init_retries, query_version_on_sync, wifi_status_on_sync,
  periodic_wifi_status, button_event_repeat_guard, local_datapoints, and an
  on_button_event trigger. Adds diagnostic accessors (last rx/tx frame, status,
  seen datapoints) for probing new models.
- Fan: speed_mappings for models whose stock levels are not 1..n, and
  configurable direction values for models using something other than 0/1.
- Malformed datapoints now skip that entry instead of abandoning the rest of
  the frame, and zero-length payloads no longer read past the buffer.

The 712S example, model notes and partition table live in docs/dreo-712s/.
Adds a .gitignore, which the repo did not previously have.

Compile-tested with ESPHome 2026.7.3: both the new 712S example and the
existing HTF018S example build clean against these component changes.