Add WiFi-only Matter (CHIP) usermod for WLED-MM

[Copilot]

WLED-MM has no Matter smart-home protocol support. Existing Matter implementations assume BLE for commissioning, which adds ~60–100KB RAM overhead and is impractical on constrained ESP32 builds. This adds Matter support using WiFi-only commissioning (Soft-AP/mDNS), bypassing BLE entirely.

Changes

• New usermod (usermods/usermod_v2_matter/): Exposes WLED as a Matter Extended Color Light (0x010D) with On/Off, Level Control, and Color Control (HSV + CT) clusters. Bridges Matter attribute callbacks to WLED's col[]/bri state via colorUpdated(CALL_MODE_DIRECT_CHANGE). Bidirectional sync — WLED UI/API/preset changes push back to the Matter fabric every 250ms.
• Thread safety: Matter SDK callbacks run on the Matter task; pending state is transferred via volatile flags and consumed in the Arduino loop() context.
• Registration: USERMOD_ID_MATTER (95) in const.h, conditional include/registration in usermods_list.cpp behind #ifdef USERMOD_MATTER.
• Build environment: [env:esp32s3_matter_wifi] in platformio.ini with -D USERMOD_MATTER -D CONFIG_BT_ENABLED=0 -D CONFIG_RENDEZVOUS_MODE_WIFI=1. Requires ESP-IDF v5.1+ with the esp_matter component.

Color mapping

Matter hue (0–254)  →  WLED hue (0–65535):  wledHue = matterHue * 65535 / 254
Matter sat (0–254)  →  WLED sat (0–255):    wledSat = matterSat * 255 / 254
Matter CT (mireds)  →  colorCTtoRGB()
WLED RGB → Matter:     standard RGB→HSV with output clamped to 0–254

Zero impact on existing builds — all Matter code is behind #ifdef USERMOD_MATTER.

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