Make the min_brightness step actionable, and cap max_brightness

min_brightness was one line with no procedure. Setting it to 1 is wrong —
triacs stop latching reliably at very low phase angles, so it presents as
flicker rather than a dim glow. Document walking it down to instability
and backing off a step, since the usable floor is fixture-specific.

Also add max_brightness, which matters more: filament life goes roughly
as V^-13 against light as V^3.4, so running at 95% roughly doubles lamp
life for ~15% less output. Worth having on a fixture whose lamp type may
be discontinued.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/stage-light-dimmer.md

@@ -172,11 +172,22 @@ One setting genuinely matters:
   means a power blip or a Z2M container restart brings a 250 W stage lamp to full
   brightness in an empty room.
 
-Two worth doing:
+Three worth doing:
 
-- **Lower `min_brightness`.** Defaults are tuned to stop LEDs flickering at the
-  bottom of the range. A filament dims smoothly to near-zero, so dropping it
-  recovers the usable low end.
+- **Tune `min_brightness` by walking it down.** The default is set to stop LEDs
+  flickering at the bottom of the range, and a filament doesn't need that
+  protection — it dims smoothly to near-zero. But don't just set it to 1: at very
+  low phase angles the triac stops latching reliably, which shows up as flicker
+  or dropout rather than a dim glow. On the Z2M device page, lower it a step at a
+  time until the lamp goes unstable, then back off one step. That point is the
+  real bottom of the usable range, and it's fixture-specific — worth writing the
+  final value down.
+- **Consider capping `max_brightness` around 95%.** Filament life is brutally
+  sensitive to voltage: roughly `life ∝ V⁻¹³` against `light ∝ V³·⁴`. Running at
+  95% buys about **double the lamp life for ~15% less light** — a good trade on a
+  fixture whose lamp type may well be discontinued (§6). At 90% it's roughly 4×
+  life for ~30% less light, if longevity matters more than output. Rules of
+  thumb, not exact, but the direction is very strong.
 - **Pass `transition: 2` in `light.turn_on` calls.** Ramping over a couple of
   seconds cuts the cold-filament inrush, which is the biggest single wear event
   on the lamp and the main stress on the triac.