Sub-pixel scrolling: motion at the frame rate, not the pixel rate

With integer positioning the number of distinct frames per second equals the
scroll speed in px/s, however fast the loop renders. Measured at 50px/s and
78.7fps, 36% of frames were byte-identical: the extra frames cost work and
bought no motion, and what was left was 50 discrete 1px steps a second.

Two things were wrong with the pre-existing sub-pixel support. get_visible_portion
never consulted sub_pixel_scrolling — it always took the integer path, so the flag
and _get_visible_portion_subpixel were dead code. And that implementation needed
scipy.ndimage.shift, which is not installed on the target devices (HAS_SCIPY is
False there), so it would not have interpolated even if reached. Verified both:
positions 1000.0 and 1000.5 produced identical frames either way.

Blending is now wired up and implemented with numpy. Two details make it
affordable: slice cached_array directly instead of building two PIL images only
to convert them straight back (the naive version measured 15x the integer path),
and use fixed-point uint16 multiply-add rather than float32, which suits the Pi's
cores and gives finer weighting than the panel can resolve. Result 0.939ms
against 0.237ms — 0.70ms added per frame, a 1065fps ceiling.

Measured on hardware: 81.2 fps with blending on, against 78.7 with it off, so no
cost within noise — and every frame is now a distinct position rather than one in
three being a repeat.

The trade is a slight horizontal softening of text, since each frame blends two
positions. Set smooth_scroll false for maximum crispness.

Also benchmarked and cleared as non-issues: extending the strip costs 9.4ms on an
11,000px strip and trimming 2.5ms, both under one frame at this rate.

Co-Authored-By: Claude <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KEZK1P1Q1fu5pcuVrkrCFZ

web_interface/templates/v3/partials/display.html

@@ -510,6 +510,17 @@
                         </label>
                     </div>
 
+                    <div class="form-group mb-4" id="setting-display-vegas_smooth_scroll" data-setting-key="display.vegas_scroll.smooth_scroll">
+                        <label class="flex items-center">
+                            <input type="checkbox"
+                                   id="vegas_smooth_scroll"
+                                   name="vegas_smooth_scroll"
+                                   {% if main_config.display.get('vegas_scroll', {}).get('smooth_scroll', True) %}checked{% endif %}
+                                   class="form-checkbox">
+                            <span class="ml-2 text-sm text-gray-700">Smooth sub-pixel motion{{ ui.help_tip('Blend between neighbouring pixel positions so the ticker moves once per rendered frame instead of once per pixel. Default: on.\nWithout it, motion happens only as often as the scroll speed in pixels per second — at 50 px/s that is 50 steps a second however fast the display renders, which reads as a slight judder. The trade is that text softens very slightly horizontally, since each frame blends two positions. Turn it off if you prefer maximum crispness.', 'Smooth Scrolling') }}</span>
+                        </label>
+                    </div>
+
                     <div class="grid grid-cols-1 md:grid-cols-2 gap-4 mb-4">
                         <div class="form-group" id="setting-display-vegas_extend_threshold_screens" data-setting-key="display.vegas_scroll.extend_threshold_screens">
                             <label for="vegas_extend_threshold_screens" class="block text-sm font-medium text-gray-700">Extend When (screens left){{ ui.help_tip('How much unscrolled content triggers loading the next group, measured in screen widths (1.0–10.0).\nDefault: 2. Higher loads earlier and leaves more slack, at the cost of holding more content in memory. Only applies when Continuous Scroll is on.', 'Extend Threshold') }}</label>