ci: add a smoke-test gate before the full test matrix

[nstarman]

Summary

• Every package (unxt itself and each packages/* member) now has a tests/smoke/ folder (fast, broad checks: imports, public-API re-exports) alongside a tests/unit/ folder (everything else)
• Add .github/workflows/ci.yml's smoke job, running right after format and gating the full multi-OS/multi-Python matrix (tests-unxt*, test-oldest, test-newest, test-interoperability)
• The smoke job collects via pytest tests/smoke packages/*/tests/smoke — proper directory-based collection, not an enumerated file list. A new package's smoke coverage is just adding the folder; no workflow edit required
• Existing test_public_api.py files (already smoke tests in substance, several already docstring-labeled "Smoke tests for...") move wholesale into smoke/ where every test in them qualifies. unxts.api and unxts.hypothesis get split, since each had one file mixing re-export checks with real behavior/property-based tests. unxts.interop.gala and unxts.interop.matplotlib — which previously had no public-API smoke check at all — get a new minimal smoke/test_public_api.py mirroring their siblings, with their existing behavior tests moved to unit/
• noxfile.py and the full per-package test jobs need no changes: they already point at whole tests/ directories, which transparently include the new subfolders
• docs and benchmark are left ungated — neither is part of "the test matrix" and both already run on their own narrower conditions

Test plan

• pytest --collect-only tests/smoke packages/*/tests/smoke collects all 24 smoke tests with no errors (confirmed locally)
• pytest tests/smoke packages/*/tests/smoke passes in ~2s locally
• Spot-checked the moved/split unit/ files (unxts.api, unxts.hypothesis, unxts.interop.gala, unxts.interop.matplotlib, unxts.linalg) run and pass
• smoke job runs and passes in CI on this PR, and the full matrix jobs correctly wait on it

[Copilot]

Pull request overview

This PR adds a new smoke GitHub Actions job intended to quickly detect “workspace is broadly broken” failures (imports / public API re-exports) before running the full multi-OS, multi-Python test matrix.

Changes:

• Introduces a smoke job that installs the workspace and runs a small set of representative tests across packages.
• Gates the full test matrix (tests-unxt*, test-oldest, test-newest, test-interoperability) on successful completion of smoke.
• Updates the final status/“CI Pass” aggregation job to include the new smoke job.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.62%. Comparing base (c5a9d8d) to head (7d53648).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##              main     #895      +/-   ##
===========================================
- Coverage   100.00%   99.62%   -0.38%     
===========================================
  Files            4       82      +78     
  Lines           25     3997    +3972     
  Branches         0      312     +312     
===========================================
+ Hits            25     3982    +3957     
- Misses           0       10      +10     
- Partials         0        5       +5     

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