Arena Studio session roles: remove 'in_trigger' from the DIO 1 selector (only DIO 2 routes to EINT)

[mbreiser]

Problem

Bench observation (2026-07-24): DIO 1 cannot serve as the external trigger for the panels' Triggered/Gated modes — only DIO 2 can. Only the "Digital IO 2 (5V)" BNC routes to the panels' EINT net (U3 B→A + J30 shunt); DIO 1 has no physical path.

Arena Studio's "Rig I/O — session roles" panel (arena_studio.html, the cIoRoleDio1 selector) still offers in_trigger for DIO 1. The gate tooltip near line 6034 even half-acknowledges the constraint ("on Digital IO 2 this is the EINT trigger route") — but the option remains selectable, the controller accepts it (companion firmware issue below), and the user ends up with a silently dead trigger.

Suggested fix (not tested — logging only for now)

• Remove in_trigger from the DIO 1 role <select> (or grey it with a "no EINT route on this port" tooltip, consistent with how roles needing missing firmware are greyed).
• Warn (or refuse) when a rig YAML io: block declares in_trigger on dio port 1 at connect-time apply.

Companion

Controller-side half: reiserlab/LED-Display_G6_Firmware_Arena#45 (SET_DIO_ROLE should refuse in_trigger on port 1 so the misconfiguration is loud).