bench report stage in/out + load, and pixi workspace config

[nclack]

Small follow-up rolling up the remaining working-tree changes left out of #155.

Two independent changes, one commit each:

• bench: per-stage in/out throughput + load — bench/report.py's stage table
  now reports GB/s_out and load% (stage ms_total / wall) alongside the
  existing GB/s_in, so each pipeline stage shows input and output throughput
  and its share of the wall — making it obvious which stage bounds a run.
• chore: pixi workspace config — [tool.pixi.*] workspace/environments in
  pyproject.toml, .pixi/* ignored in .gitignore, and pixi.lock merge
  attributes in .gitattributes.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 57.73%. Comparing base (cca3b09) to head (c7595d8).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #156      +/-   ##
==========================================
- Coverage   57.78%   57.73%   -0.06%     
==========================================
  Files          64       64              
  Lines       10046    10055       +9     
  Branches     1748     1750       +2     
==========================================
  Hits         5805     5805              
- Misses       3494     3500       +6     
- Partials      747      750       +3     

Flag	Coverage Δ
unittests	57.73% <ø> (-0.06%)	⬇️

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see 5 files with indirect coverage changes

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