Add automatic EDID-based mode switching

[drhalftone]

Summary

• Implement automatic HDMI connection detection based on EDID reads
• Add pixel clock watchdog timer for disconnection detection
• Maintain manual override capability via DIP switch

Problem

The Mimas A7 board has PCB crosstalk issues that cause ghost HDMI clock signals even when no cable is connected. This previously required manual mode switching via DIP switch #4.

Solution

This PR implements automatic mode switching by:

1. Detecting when a PC reads the EDID data (indicates real connection)
2. Monitoring pixel clock presence with a 2-second watchdog timer
3. Automatically switching between pass-through and offline modes

Implementation Details

• edid_rom.vhd: Added edid_read_detected output that pulses when EDID is read
• hdmi_io.vhd: Added watchdog timer and mode switching logic
• Manual override via DIP switch #4 is preserved for backward compatibility
• Automatic mode status is exported on the sel signal for debugging

Test Plan

• Connect HDMI cable from PC - verify automatic switch to pass-through mode
• Disconnect HDMI cable - verify automatic switch to offline mode after 2 seconds
• Test manual override with DIP switch #4
• Verify 800x600@120Hz generation in offline mode
• Test reconnection scenarios

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