Update README IRX paths and examples

[NathanNeurotic]

Summary

• Correct external IRX lookup order to use SYS-CONF root without PS2BBL subdirectories on memory cards
• Clarify IRX placement in example configuration and required files
• Add additional CONFIG.INI examples covering common device combinations and fallback strategies
• Align runtime IRX lookup code with the documented SYS-CONF search paths

Testing

• Not run (documentation-only changes)

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