Add downgrade CI for all sublibraries

[ChrisRackauckas]

Summary

• Adds comprehensive downgrade CI testing for all sublibraries
• Tests the main package (.) and all 10 sublibraries in
• Each sublibrary is tested individually with downgraded dependencies
• Uses julia-downgrade-compat@v2 with ALLOW_RERESOLVE: false

Sublibraries Covered

Main package:

10 Sublibraries:

Benefits

• Ensures compatibility across the entire package ecosystem
• Catches dependency issues early for each sublibrary
• Provides comprehensive downgrade testing coverage
• Maintains lean testing with focused sublibrary-specific tests

Test plan

• Verify workflow triggers correctly on PRs and pushes
• Confirm each sublibrary builds and tests successfully
• Check that downgrade testing works for all projects
• Validate matrix strategy runs all combinations

🤖 Generated with Claude Code

[ChrisRackauckas]

Fix for Reexport dependency conflict

I've pushed a fix for the downgrade CI failure. The issue was:

Problem:

• When running with ALLOW_RERESOLVE: false in the downgrade CI, Julia's package resolver cannot automatically update dependencies
• The project files had Reexport = "1" or Reexport = "1.2" which gets pinned to exactly v1.2.0
• However, OrdinaryDiffEqTsit5 v1.2.0 requires Reexport v1.2.2
• This created an unsatisfiable constraint

Solution:
Updated all Reexport version constraints to "1.2.2" across all 8 Project.toml files:

• Main project
• All 7 sublibraries that depend on Reexport

This ensures compatibility with OrdinaryDiffEqTsit5's requirements when running the downgrade CI tests.

🤖 Generated with Claude Code