Run downgrade workflow on main

[ChrisRackauckas-Claude]

Summary

• run the downgrade workflow after pushes to the repository's actual default branch, main
• leave the existing pull-request trigger and reusable workflow unchanged

The workflow currently listens for pushes to master, so it has no completed
run at the current default-branch SHA.

Local validation

• exact Julia 1.10 downgrade action, build, and locked test replay: passed
• Core/core_tests.jl: 172201/172201 passed
• actionlint .github/workflows/Downgrade.yml: passed
• Runic 1.7 over all 22 Julia files: passed
• git diff --check: passed

Ignore this PR until it has been reviewed by @ChrisRackauckas.

[ChrisRackauckas-Claude]

Commit 3462078 changes only the downgrade push trigger from master to main. Local verification: exact Julia 1.10 downgrade/build/locked Core replay passed (172201/172201), actionlint passed, Runic checked all 22 Julia files, and git diff --check passed.