Rename threadCPURatio to threadCPUPercent and make it a Uint8Array

[mstange]

This turns it into a Uint8Array of integer percentages, which requires less memory bandwidth when rendering the activity graph.

We now always have a fallback threadCPUPercent array, even if the original profile doesn't contain any threadCPUDelta information. The synthesized values are set to 100%.

We also have a trailing extra value in order to simplify the rendering code for the last sample.

There's one behavior change related to the one-past-the-end value: In the past, we said "after the last sample, continue with the same CPU percentage as just before the last sample". Now we say "after the last sample, the CPU percentage is zero." The profile from the 'matches the 2d canvas draw snapshot with only one CPU usage value' test demonstrates this:
Main branch | Deploy preview

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.41%. Comparing base (cb9c976) to head (f3f07c7).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5898      +/-   ##
==========================================
- Coverage   85.42%   85.41%   -0.01%     
==========================================
  Files         321      321              
  Lines       32168    32157      -11     
  Branches     8785     8865      +80     
==========================================
- Hits        27479    27468      -11     
  Misses       4253     4253              
  Partials      436      436              

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[canova]

LGTM, thanks!