Simplify downgrade CI on Julia 1.12

[Krastanov-agent]

Summary

• run lower-bound resolution and testing on Julia 1.12
• resolve root and split test environments together, then build and test the locked graph
• disable both Pkg.test re-resolution paths
• remove pre-1.12 setup, manifest-repair, workspace-rewrite, and direct-test workarounds
• retain strict forcedeps verification and only the required stdlib exclusions

No compat bounds are added to test/Project.toml.

Validation

• actionlint
• all Project TOML files parsed with Julia 1.12
• aggregate workflow-contract and stdlib-skip checks
• targeted locked-manifest runs on representative and exceptional repositories
• two independent cross-repository reviews

Existing package lower-bound failures are intentionally left visible for separate follow-up.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 33.18%. Comparing base (13933af) to head (4e48fc6).
⚠️ Report is 8 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #90      +/-   ##
==========================================
+ Coverage   32.95%   33.18%   +0.22%     
==========================================
  Files          15       15              
  Lines         443      443              
==========================================
+ Hits          146      147       +1     
+ Misses        297      296       -1     

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