Accept stable residual pressure when a zero target cannot be reached exactly

[SebRoLENS]

Zero-target steps previously depended entirely on the instrument's IN LIMIT flag and could time out while pressure had stopped falling near zero. Add an alternative plateau criterion only for an exact zero target: residual within ±0.5 bar, pressure range ≤0.01 bar over 10 seconds, with valid readings and no telemetry gap above three polling intervals (at least three seconds).

Manual, indenting and routine steps can complete or begin their configured dwell using this condition. Nonzero targets keep the hardware IN LIMIT criterion. Telemetry failures, CONTROL exit, disconnection and new pressure steps clear the stability history. UI shows ZERO STABLE / ZERO STABILE while preserving the actual pressure and hardware IN LIMIT flag; diagnostic logs record the accepted residual.

Validation: 46 cross-platform tests and 7 repository checks pass locally; all workflows pass actionlint. New tests cover a stable 0.5 bar residual, continuing ramps, residuals outside tolerance, nonzero targets, missing/invalid readings, dwell/completion and real polling-to-telemetry integration. Instrument validation remains to be performed.