Pace the Vegas frame loop adaptively: 31.5 -> 78.7 fps

The loop slept a fixed frame_interval on top of however long the frame took, so
at a measured 31.6ms per frame a flat 8ms of that was pure idle — a quarter of
the budget spent not rendering. It now sleeps only the remainder of the budget.

Measured on hardware: 31.5 fps to 78.7 fps sustained, with CPU going *down* from
150% to 127%. Scroll speed is unchanged at 49.9px/s against a configured 50,
because motion is derived from elapsed time rather than frame count — this buys
smoothness, not speed.

Worth recording what the bottleneck was not: the per-frame render path measures
0.34ms in total (0.18ms for the numpy slice, 0.17ms for the dirty-tracking
digest), which is a theoretical 2900 fps. Optimising any of that would have been
wasted effort. The frame was idle, not busy.

Also nices the prefetch thread. Its work is PIL and numpy that releases the GIL,
so the scheduler can act on the priority, and without it the prefetch competes
for the same cores as the render loop.

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

src/vegas_mode/render_pipeline.py

@@ -6,6 +6,7 @@ Uses the existing ScrollHelper for numpy-optimized scroll operations.
 """
 
 import logging
+import os
 import time
 import threading
 from collections import deque
@@ -256,6 +257,15 @@ class RenderPipeline:
                 return  # already have one waiting
 
             def _work():
+                # Deprioritise against the render loop. Linux applies nice
+                # per-thread, and the heavy lifting here is PIL and numpy work
+                # that releases the GIL, so the scheduler can actually act on
+                # it — without this the prefetch competes for the same cores and
+                # costs frames.
+                try:
+                    os.nice(10)
+                except (OSError, AttributeError):
+                    pass
                 try:
                     group = self.stream_manager.take_next_group(offscreen_only=True)
                 except Exception: