Spectrogram: stop dragging the labels along with the picture

The spectrogram scrolled by copying the visible canvas onto itself one step
along. That canvas already held the frequency labels and gridlines, because
they were drawn straight onto it, so every frame smeared them sideways and
left a trail. Arrived with the labels in 0.3.0; the gridlines did it too.

Only measured pixels belong on a surface that scrolls. The history now lives
on its own offscreen canvas that holds nothing but spectrum, and the visible
canvas is repainted from scratch each frame — history first, annotations
over the top. Nothing drawn as an annotation can reach the scroll buffer by
construction, rather than by remembering to clear the right strip.

The layout was muddled in the same place and is fixed with it: the gutter
used to be reserved along the bottom while the labels were drawn down the
left, overlapping the plot. Labels now sit beside the axis they describe —
down the left when frequency runs vertically, with room for "12.5k", and
along the bottom when it runs horizontally.

Verified both orientations by fingerprinting the gutter and the plot two and
a half seconds apart: gutter identical, plot changed.

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

modules/spl-meter/module.json

@@ -1,7 +1,7 @@
 {
   "id": "spl-meter",
   "name": "SPL Meter",
-  "version": "0.4.0",
+  "version": "0.4.1",
   "proddash": ">=1.5.0",
   "description": "Calibrated sound level, Leq and RTA from an audio input on this machine",
   "permissions": [