use parallel mode for coverage

[mehrdad2m]

Context:
make lit-coverage runs lit with -j$(nproc) while every worker does coverage run --append into one shared .coverage.lit. That file is a SQLite database and does not tolerate concurrent writers, so workers intermittently abort.

Description of the Change:

• frontend/test/lit/lit.cfg.py: --append → --parallel-mode, so each worker writes its own .coverage.lit.<host>.<pid>.<random>

• Makefile: merge them at the end of lit-coverage with coverage combine --data-file=$(MK_DIR)/.coverage.lit

Benefits:

Possible Drawbacks:

Related GitHub Issues:

Assisted by: Claude Opus 5

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.62%. Comparing base (e0bf6d6) to head (1261917).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3215      +/-   ##
==========================================
- Coverage   97.23%   95.62%   -1.61%     
==========================================
  Files          40      177     +137     
  Lines        4522    20877   +16355     
  Branches        0     2102    +2102     
==========================================
+ Hits         4397    19964   +15567     
- Misses        125      725     +600     
- Partials        0      188     +188     

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[paul0403]

🥳

[joeycarter]

In Opus 5 we trust.

(But yes, this solutions agrees with the description in the CodeCov docs).