[EPM-3469] Add ILI9488 TFT driver, GPIO monitor, UI dashboard, and LCD backlight on boot

[vmanchala-dev]

Summary

Adds the ILI9488 TFT display stack and turns on the LCD backlight at boot on device targets.

New modules

• Ili9488.Driver (ili9488_driver.ex) – SPI/GPIO setup, reset, init, and drawing (fill, rectangles) for the ILI9488 (320×480, RGB666).
• Ili9488.GPIOMonitor (ili9488_gpio_monitor.ex) – Monitors GPIO 71 every 500 ms and updates the on-screen indicator (green = high, grey = low).
• Ili9488.UI (ili9488_ui.ex) – Draws the dashboard: blue background, textbox, "GPIO71" label, and GPIO indicator.
• TestC2.LcdBacklight (lcd_backlight.ex) – Turns on the LCD backlight (GPIO 116) at application start; supervised on device targets only (not host).

Other changes

• Application: TestC2.LcdBacklight added to target_children() so the backlight is on at boot on device.
• Docs: docs/ILI9488_MODULES.md with module overview, API, how to run, and testing GPIO 71 toggle from the console.
• README: Boot behavior (LCD backlight), ILI9488 section with link to full docs.

How to run (IEx)

{:ok, devs} = Ili9488.Driver.start()
Ili9488.UI.draw_dashboard(devs)
Ili9488.GPIOMonitor.start(devs)

Toggle GPIO 71 from console to see indicator change:

{:ok, g} = Circuits.GPIO.open(71, :output)
Circuits.GPIO.write(g, 1)  # green
Circuits.GPIO.write(g, 0)  # grey

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Note

Medium Risk
Directly manipulates SPI/GPIO and adds a supervised boot-time hardware side effect (backlight), which could impact device startup or interfere with pin assignments if misconfigured.

Overview
Adds an initial ILI9488 TFT display stack: a new Ili9488.Driver for SPI/GPIO init + basic drawing, a simple Ili9488.UI dashboard (blue background, textbox, GPIO71 label + indicator), and an Ili9488.GPIOMonitor process that polls GPIO 71 every 500ms and updates the indicator on changes.

Enables the LCD backlight at device boot by adding a supervised TestC2.LcdBacklight GenServer (GPIO 116 driven high) to target_children() (targets only). Documentation is updated with boot/display behavior plus a new docs/ILI9488_MODULES.md usage guide.

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