CI: split test job into parallel query-engine and reporting shards

[tombonfert]

Summary

Splits the single test job in acceptance.yml into a parallel matrix over
component: [query_engine, reporting], so the two halves of the suite run
concurrently on separate runners instead of back-to-back. The shared
not-a-fork → lint gate is unchanged.

• Makefile: test target takes an optional TEST_PATH (defaults to
  tests/); CI passes each component's path. Coverage still measured against
  all of src/.
• acceptance.yml: test becomes a fail-fast: false matrix; each shard
  uploads coverage under a Codecov flag. Replaces the old single-value
  python-version matrix.
• codecov.yml: declares query_engine / reporting flags with
  carryforward so per-shard uploads merge into the combined total.

Verified the partition is exact (780 + 598 = 1378 tests, no overlap) and that
a sharded make test still emits coverage.xml.

Test Plan

• Unit tests added/updated
• Manual testing completed
• Documentation updated (if applicable)

Checklist

• Code follows project style guidelines
• Self-review completed
• No new linter warnings introduced

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.45%. Comparing base (f654d22) to head (a644d2c).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #70      +/-   ##
==========================================
- Coverage   88.72%   88.45%   -0.27%     
==========================================
  Files          59       59              
  Lines        4826     4826              
  Branches      552      552              
==========================================
- Hits         4282     4269      -13     
- Misses        445      455      +10     
- Partials       99      102       +3     

Flag	Coverage Δ
query_engine	84.04% <ø> (?)
reporting	94.31% <ø> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.
see 5 files with indirect coverage changes

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