v0.42.0: laser speaker range, phosphor afterimage; line width box, radial dot size

Laser: a two-handle "Speaker range" sets what the speaker under the mirror
plays (36 dB/octave edges; the noise floor is measured per range so treble
alone still shows). Bass-only gives clean loops; treble-only is a shimmer
along one line, as a membrane far above its resonances really draws.

The afterimage is now a phosphor buffer, dimmed each frame and rounded
down so it always fades to nothing — constant cost however long the fade
(drawing every past sweep ran the big window past a frame).

Line width is a number box stepping by 0.5; radial dots have a size slider.

CLAUDE.md

@@ -198,9 +198,22 @@ persistence) → GUI (Qt widgets that read the manager and connect to its signal
   `distance`, turned by pitch/yaw/roll; the auto-gain holds the beam
   *angle*, so distance is real, and `scale()` shrinks the drawing only when a
   steep screen would push the loudest swing out of view. The path is drawn
-  brighter where it moves slower (dwell); past sweeps are kept for the
-  afterimage time as pre-built polygons in screen metres, drawn through a
-  transform with a cosmetic pen. Every style's fade is one seconds slider,
+  brighter where it moves slower (dwell). The afterimage is a **phosphor**
+  (`_Phosphor`, one per painted size, half resolution past 400 px wide):
+  each sweep is burned in once and the buffer is dimmed per frame, so cost
+  doesn't grow with the fade time. The dimming is a numpy LUT that rounds
+  *down*, because Qt's own fade rounds to nearest and stalls faint pixels
+  forever. The numpy array must be a **view of the QImage's `bits()`**, since
+  building a QImage *from* an array copies it. A **speaker range** (low/high
+  Hz, `gui/range_slider.py`) filters what drives the membrane, with
+  `one_pole` stages done exactly via the cumsum trick, 36 dB/octave. That is
+  steeper than a real speaker on purpose: the membrane answers bass ~80 dB
+  more than treble, so a gentler slope lets bass drown a treble-only
+  setting. For the same reason the auto-gain floor is *measured* per range
+  (reference hiss through that speaker and membrane), and the membrane is
+  simulated at full rate only once the bass is cut. Treble alone draws a
+  shimmer along one line: far above every resonance, all modes move in
+  phase. Every style's fade is one seconds slider,
   0 = off. **Sync** (Round 78): decoding runs ahead of the speakers, by a
   buffer or two normally but by up to ~0.8 s when the GUI thread stalls and
   buffers arrive in a burst. So the widget keeps 1.5 s of *timestamped*