fix(spoolbuddy): gate display wake on stable scale reading
A noisy load cell that bounced ≥50 g around its midpoint kept the kiosk
screen permanently lit. The wake threshold check ran against last_wake_grams
which itself advanced to noisy values, so every bounce back across 50 g
re-fired display.wake(), keeping the FIFO reader pumping wlopm --on
faster than swayidle could trigger wlopm --off.
The fix gates wake on the scale's `stable` flag — only readings that
have settled within 2 g over a 1 s window count as a real spool
placement / removal. Unstable noise can't fire wake and can't advance
last_wake_grams, so the next genuine settled change is still measured
against the right baseline.
Three regression tests pin the contract: noisy ±60 g unstable readings
never wake, a settled >50 g jump wakes, a noise burst between two
settled readings doesn't poison last_wake_grams.
maziggy