trial-params gain widen (int16) + controller-run duration (arena-fw #4)

[floesche]

Match the firmware's canonical trial_params (0x08) re-layout from reiserlab/LED-Display_G6_Firmware_Arena#4: gain widens from int8 to int16 and moves after init_pos, and a new duration field lets the controller auto-stop a trial on its own instead of relying on a host follow-up command.

• encodeTrialParams: reordered params (init_pos @5-6, gain @7-8, duration @9-10), gain now uses the existing i16le helper (int8 helper dropped), duration stays in seconds at the JS API boundary and converts to 10 ms ticks on the wire (AC::constants::duration_tick_ms, matching the firmware)
• buildTrialParams: carries duration through from the trialParams command, same convention as conditionDuration's wait/trialParams math
• hostSideTrialEnd/ArenaRunner._timer/the setTimeout mechanism is untouched — host-side timing stays authoritative for now; the wire-side duration just unblocks that swap for later, it isn't made here (kept the blast radius small until this is bench-verified)
• golden-vector tests rewritten for the new offsets and widened gain range, plus new duration vectors and range-throw cases

This is a breaking wire change — bytes 5-10 of the 0x08 payload are reinterpreted — and ships in lockstep with the companion Arena-Firmware PR. A stale pairing would silently misparse gain/init_pos/duration into garbage that still returns status == 0.

Verification

• pixi run test: full suite passes, including the rewritten trial-params golden vectors in tests/test-arena-wire-g6.js and tests/test-arena-runner-g6.js
• pixi run format-check
• Bench-verified against the paired Arena-Firmware branch: fed encodeTrialParams()'s actual output bytes to a Teensy 4.1 over serial and confirmed Mode 2 (initPos=9, gain=-20, duration=1.5s) applies init_pos correctly and auto-stops to ALL_OFF on its own, and Mode 4 gain=2000 (past the old int8 ceiling) is accepted

Test plan

• pixi run test
• pixi run format-check
• Hardware round-trip with Arena-Firmware gh4-trial-params-relayout
• Merge together with the Arena-Firmware PR, not independently

This is part of the reiserlab/LED-Display_G6_Firmware_Arena#35 PR