feat(scroll): show which scroll speeds are smooth on this panel (#710)

* feat(scroll): show which scroll speeds are smooth on this panel

The Vegas Scroll Speed slider now says what the panel will do with the
chosen speed and offers the nearest smooth ones to click. Backed by
scroll_config.speed_advice() and GET /api/v3/config/scroll-speed-advice,
which uses the refresh the display measured rather than the cap.

Also stops the default 50 px/s snapping to a stepped 48 px/s (2px every 5
refreshes, 24fps) on a 120Hz panel: the low-fps penalty in solve_crisp()
now loses to 60 or 40 px/s. 100Hz panels are unchanged.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

* fix(scroll): hint threw before its timer variables existed; count 25-30fps as stepped

The Vegas speed hint called refreshScrollSpeedHint() before the let
declarations it uses, so it never rendered (found on ledpi). And the
solver's low-fps penalty stopped at 25fps, which let a measured 125.7Hz
panel keep a 25.1fps 2px-every-5-refreshes scroll.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

* test: add the scroll-speed-advice route to the /api/v3 URL map snapshot

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Sonnet 5.5 <noreply@anthropic.com>

web_interface/blueprints/api_v3/config.py

@@ -84,6 +84,49 @@ def get_main_config():
     # any of it; /api/v3/auth/* manages it.
     return jsonify({'status': 'success',
                     'data': _redact_credentials(strip_auth_section(config))})
+def _panel_refresh_hz(config):
+    """(hz, source): the rate the panel really refreshes at, else the cap.
+
+    The display service writes what it measured to the frame-stats file. The
+    configured limit_refresh_rate_hz is only a cap (a 120Hz cap refreshes at
+    ~126Hz on one rig, ~95Hz on a long chain), and the speeds that look smooth
+    are fractions of the real rate, so advice built on the cap can be wrong.
+    """
+    from src.common import frame_timing, scroll_config
+    cap = scroll_config.refresh_hz_from_config(config)
+    try:
+        with open(frame_timing.default_stats_path(), encoding='utf-8') as fh:
+            measured = float(json.load(fh).get('measured_refresh_hz') or 0)
+    except (OSError, ValueError, TypeError, AttributeError):
+        measured = 0.0
+    # Reject a stale file from a previous hardware config: a measurement far
+    # off the cap says the config changed since it was written.
+    if measured > 0 and 0.5 * cap <= measured <= 1.5 * cap:
+        return measured, 'measured'
+    return cap, 'configured'
+
+
+@api_v3.route('/config/scroll-speed-advice', methods=['GET'])
+def get_scroll_speed_advice():
+    """What this panel does with a requested scroll speed, and smooth options.
+
+    Backs the hint under the Vegas Scroll Speed slider.
+    """
+    from src.common import scroll_config
+    try:
+        speed = float(request.args.get('speed', ''))
+        lo = float(request.args.get('min', 10))
+        hi = float(request.args.get('max', 200))
+    except ValueError:
+        return jsonify({'status': 'error', 'message': 'speed must be a number'}), 400
+    if not api_v3.config_manager:
+        return jsonify({'status': 'error', 'message': 'Config manager not initialized'}), 500
+    hz, source = _panel_refresh_hz(api_v3.config_manager.load_config())
+    advice = scroll_config.speed_advice(speed, hz, lo, hi)
+    advice['refresh_source'] = source
+    return jsonify({'status': 'success', 'data': advice})
+
+
 @api_v3.route('/config/schedule', methods=['GET'])
 def get_schedule_config():
     """Get current schedule configuration"""