[P403n1x87]

We fix how asyncio task data is read and written, and add support for the running task of a sample.

Task time used to be attached to every sample's task tree, so the same report was counted again at every sample that followed. Task trees now only hold structure, and each task report is attached once to the sample it came with, together with the task's previous position.

Task reports no longer carry time on the wire: they are delimited by their frame count, and the reader works out the time a task spends at a position from its thread's samples while it holds it without running (none in CPU mode). At the end of a stream, every live task gets a report with the time it has spent at its position since its last one.

Tasks that are gone are now forgotten together with their waiter edges, a task's waiter set no longer merges with an earlier one across samples, a waiter of 0 empties it, and a waiter that isn't known no longer hides the tasks it awaits.

MOJO_TASK_RUNNING gives the task a sample's thread was running at that sample, as AustinSample.running_task. The writer writes task reports as they were read and only the waiter edges that changed, and compressed task ids start at 1, 0 standing for no task.

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[codecov]

Codecov Report

❌ Patch coverage is 94.39252% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.14%. Comparing base (059e78c) to head (7cf2f43).

Files with missing lines	Patch %	Lines
austin/format/mojo.py	93.68%	3 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #57      +/-   ##
==========================================
+ Coverage   78.61%   79.14%   +0.53%     
==========================================
  Files          20       20              
  Lines        1987     2057      +70     
  Branches      246      256      +10     
==========================================
+ Hits         1562     1628      +66     
- Misses        353      355       +2     
- Partials       72       74       +2     

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