feat: add Waveshare ESP32-C6-LCD-1.47 board support

[MainzelMennchen]

Summary

Adds support for the Waveshare ESP32-C6-LCD-1.47 board:

• ESP32-C6, Single-Core RISC-V 160 MHz, 4 MB Flash, no PSRAM
• ST7789 display, 172×320 px (landscape 320×172), SPI
• WS2812 RGB LED on GPIO8, BOOT button on GPIO9

What's included

• New PlatformIO environment ESP32-C6-LCD-147 using pioarduino platform (required for Arduino Core 3.x / ESP32-C6)
• New device header waveshareC6LCD147.h and display driver waveshareC6LCD147Driver.cpp with landscape two-column layout
• IDF 5.x / Arduino Core 3.x compatibility fixes:
  • TFT_eSPI: Route ESP32-C6 through the C3 processor path (TFT_eSPI_ESP32_C3.h/.c) with SPI_UPDATE register sync and correct SPI_PORT=2 register addressing. Fix MISO pin conflict on C6.
  • mbedtls 3.x: _ret suffix compat macros for sha256 functions
  • WDT: esp_task_wdt_init() config struct for IDF 5.x
  • NTP: Replace NTPClient with configTime() for IDF 5.x lwIP threading
  • I2C: Disable legacy I2C driver on C6 (not needed, conflicts with IDF 5.x)
  • Mining: Software SHA256 only (no HW SHA on C6), single-core task pinning via APP_CORE macro

Tested

• Mining works at ~25 KH/s (software SHA256 on single-core 160 MHz RISC-V)
• WiFi, Stratum pool connection, configuration persistence all working
• Display renders correctly in landscape with two cyclic screens (Miner + Clock)
• Verified: TTGO-T-Display (ESP32), ESP32-2432S028R (ESP32), ESP32-C3-spotpear (ESP32-C3) all still build successfully — TFT_eSPI changes are guarded by CONFIG_IDF_TARGET_ESP32C6

Test plan

• Build ESP32-C6-LCD-147 environment
• Flash and boot on Waveshare ESP32-C6-LCD-1.47 hardware
• Verify display output (landscape, both screens)
• Verify mining (pool connection, share submission)
• Verify existing boards still compile (ESP32, ESP32-C3, ESP32-S3)