fix(scroll): tie the frame hold to the scroll, not the plugin

The hold applied in configure() never reached the panel. Plugins share one
display manager, and set_scrolling_state(False) -- fired whenever ANY other
plugin finishes its scroll -- reset the hold to 1. A hold set once at plugin
construction was therefore always gone by the time that plugin rendered.

The symptom was a log line that lied. ledmatrix-stocks reported

    Scroll configured: 50.0 px/s (1px every 2 refreshes = 50.0 fps, smooth)

while the panel measured 100.0 fps, median 10.00ms. Config, resolution and
snapping were all correct; only the pacing silently was not applied.

set_scrolling_state(is_scrolling, frame_hold=1) now carries it, so the hold
lives exactly as long as the scroll that asked for it. configure() reports the
value as ScrollSettings.frame_hold instead of applying it -- applying it behind
the caller's back could never have been right on a shared display manager.
Existing callers are unaffected; the default keeps one frame per refresh.

Verified on hardware: stocks at 50 px/s now measures

    50.0 fps over 251 frames | median 20.00ms p95 20.09ms | stalls 0 skips 0

20.00ms being exactly two refreshes, with the panel still refreshing at 100Hz
underneath so flicker is unchanged.

test_another_plugin_stopping_does_not_strand_a_hold pins the interaction that
broke this.

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

test/test_display_dirty_tracking.py

@@ -272,3 +272,43 @@ class TestFrameHold:
     def test_unusable_holds_are_ignored_or_floored(self, dm, bad, expected):
         dm.set_frame_hold(bad)
         assert dm._frame_hold == expected
+
+
+class TestFrameHoldLifetime:
+    """The hold must last exactly as long as the scroll that asked for it.
+
+    Plugins share one display manager. A hold applied at plugin construction is
+    wiped the moment any *other* plugin finishes scrolling, so by the time the
+    first plugin renders it is back to one pixel per refresh -- the speed reads
+    correct in the log and is wrong on the panel.
+    """
+
+    def test_scrolling_state_carries_the_hold(self, dm):
+        dm.set_scrolling_state(True, frame_hold=4)
+        try:
+            dm.draw.rectangle([0, 0, 7, 7], fill=(10, 200, 10))
+            with _SwapSpy(dm.matrix) as spy:
+                dm.update_display()
+            assert spy.last_frame_hold == 4
+        finally:
+            dm.set_scrolling_state(False)
+
+    def test_another_plugin_stopping_does_not_strand_a_hold(self, dm):
+        dm.set_scrolling_state(True, frame_hold=3)
+        dm.set_scrolling_state(False)          # some other plugin finishes
+        dm.set_scrolling_state(True)           # a plugin that wants no hold
+        try:
+            dm.draw.rectangle([0, 0, 6, 6], fill=(200, 10, 10))
+            with _SwapSpy(dm.matrix) as spy:
+                dm.update_display()
+            assert spy.last_frame_hold == 1
+        finally:
+            dm.set_scrolling_state(False)
+
+    def test_default_keeps_previous_behaviour(self, dm):
+        """Callers that never heard of frame holds get one frame per refresh."""
+        dm.set_scrolling_state(True)
+        try:
+            assert dm._frame_hold == 1
+        finally:
+            dm.set_scrolling_state(False)