[boronine-bot]

Summary

Add BRANCH_MIGRATION.md, linked from the README, with a staged migration checklist for Alexei. This PR prepares the rename without changing repository settings or renaming any branch.

The existing test workflow has no branch filters, Dependabot has no target-branch override, and the badges do not specify a branch. No existing branch-specific code/configuration changes are needed on master.

Pending PR #81 is a separate dependency: its proposed publishing workflow and release instructions hard-code master. The checklist identifies those locations and provides default-branch-independent workflow expressions. They must be reconciled before enabling publishing; this PR does not modify the other PR.

Manual steps for Alexei

1. Merge this preparation PR and review newly merged changes for branch-name references.
2. Coordinate PR Prepare OIDC-based PyPI pre-release publishing #81: use the default branch dynamically in its workflow and instructions, or update its master references to main after the rename. Do not publish during the transition.
3. Record/review branch protections, organization rulesets, external integrations, and deployment restrictions.
4. Use GitHub's Rename branch operation to rename master to main, then verify the default branch and open-PR bases were updated.
5. Verify protections and required checks still apply. If the pypi environment exists, change its explicit allowed branch from master to main while retaining required reviewers. PyPI Trusted Publisher fields need no branch-only change.
6. Update local clones and forks using the commands in the checklist, and fix external raw URLs or Git/action references that still name master.
7. Verify CI, manual workflow availability, badges, and publishing restrictions. Do not dispatch a publishing workflow merely to test this rename.

See GitHub's branch-renaming documentation.

Validation

• Audited tracked files for existing master references: none before this checklist.
• Verified the README's checklist link resolves.
• Confirmed the current upstream default branch is master and no upstream main branch currently exists.
• Repository rulesets API returned an empty list. The branch-protection endpoint returned 404, which is not treated as proof that all protections or organization policies are absent; the checklist requires administrator verification.
• git diff --check: passes.

Documentation-only change; no branch rename, configuration write, publication, or release was performed.