Prepare the default-branch rename from master to main

BRANCH_MIGRATION.md

@@ -0,0 +1,105 @@
+# Renaming the default branch from master to main
+
+This checklist prepares the rename; merging it does **not** rename a branch,
+change GitHub settings, or publish a release. An administrator such as Alexei
+must perform the GitHub steps below.
+
+## Before the rename
+
+- [ ] Merge this checklist into the current default branch.
+- [ ] Review any changes merged since this checklist was written for explicit
+  `master` references. The existing test workflow has no branch filters,
+  Dependabot has no `target-branch`, and the README badges do not specify a
+  branch, so those configurations already work with `main`.
+- [ ] Coordinate the pending [PyPI publishing setup PR #81](https://github.com/hsluv/hsluv-python/pull/81).
+  Its initial version hard-codes `master` in the build-job condition, the tag
+  ancestry check, and `RELEASING.md`. Do not assume GitHub will rewrite these.
+  Before enabling publishing, either update that PR to use the repository's
+  default branch dynamically, or change those references to `main` after the
+  rename. If it has already merged, make the equivalent follow-up change.
+  A branch-independent implementation can use:
+
+  ```yaml
+  if: github.repository == 'hsluv/hsluv-python' && github.ref == format('refs/heads/{0}', github.event.repository.default_branch)
+  ```
+
+  and pass `github.event.repository.default_branch` through an environment
+  variable for the ancestry check:
+
+  ```yaml
+  env:
+    DEFAULT_BRANCH: ${{ github.event.repository.default_branch }}
+  run: git merge-base --is-ancestor HEAD "origin/$DEFAULT_BRANCH"
+  ```
+
+  Keep the upstream-repository restriction, pre-release checks, and approval
+  requirements intact. Update the release instructions to select the default
+  branch in the workflow-dispatch UI.
+- [ ] Review branch protection, repository/organization rulesets, required status
+  checks, environment deployment restrictions, webhooks, external CI, and any
+  scripts outside this repository. Record the current settings before changing
+  them; not all are visible to contributors or represented in Git.
+- [ ] Confirm that no existing `main` branch conflicts with the rename. Do not
+  overwrite or delete an existing branch to make room without investigating it.
+- [ ] Avoid publishing or approving a release while the rename is in progress.
+
+## Administrator steps on GitHub
+
+1. Open the repository's [branches page](https://github.com/hsluv/hsluv-python/branches).
+   Use the branch menu's **Rename branch** operation to rename **`master`** to
+   **`main`**. Rename the existing branch rather than creating a second branch
+   and deleting the first. Organization rules may require an organization
+   administrator to authorize this.
+2. Verify that **Settings → General → Default branch** now names **`main`**.
+   GitHub normally updates branch protection policies and the base branch of
+   open PRs when renaming a branch; verify those changes instead of assuming
+   every rule or external integration was updated.
+3. Verify that open PRs target `main`, including PRs from forks, and that their
+   checks still run. Reconcile conflicts with the new base as needed.
+4. If the **`pypi`** environment from PR #81 exists, inspect **Settings →
+   Environments → pypi → Deployment branches and tags**. Replace any explicit
+   **branch `master`** restriction with **branch `main`** and retain required
+   reviewers. Do not remove deployment restrictions just to get a run working.
+5. The proposed PyPI Trusted Publisher identifies the repository, workflow
+   filename, and environment—not the default branch—so those PyPI fields do
+   not need changing solely for this rename. Keep `publish-prerelease.yml` and
+   `pypi` unchanged, and verify the actual configured publisher still matches.
+6. Update external integrations, raw-content URLs, scripts, or consumers that
+   explicitly request `master`. GitHub redirects ordinary file URLs, but not
+   raw file URLs, Git fetch/pull requests, or action references such as
+   `uses: owner/repository@master`.
+
+## Updating local clones
+
+First ensure the working tree is safe to switch branches and that no unrelated
+local branch named `main` already exists. With `origin` pointing to
+`hsluv/hsluv-python`, run:
+
+```sh
+git branch -m master main
+git fetch origin
+git branch --set-upstream-to=origin/main main
+git remote set-head origin -a
+```
+
+These commands rename the local default branch; feature branches need not be
+renamed. If your upstream repository is named `upstream` rather than `origin`,
+substitute that remote in the commands. Fork owners should also check their
+fork's default branch and update it separately if necessary. There is no need
+to reset commits, force-push, or discard local work.
+
+## Final verification
+
+- [ ] `gh repo view hsluv/hsluv-python --json defaultBranchRef` reports `main`.
+- [ ] The renamed branch points to the expected commit history.
+- [ ] Existing protection/ruleset requirements and required checks still apply.
+- [ ] A push or PR runs the test workflow successfully on Python 3.11 and 3.15.
+- [ ] The test workflow is available for manual dispatch from `main`; scheduled
+  workflows and Dependabot continue to use the repository's default branch.
+- [ ] README badges and links still work.
+- [ ] Any publishing workflow and release instructions now use `main` or discover
+  the default branch dynamically, and the `pypi` environment allows `main` only.
+- [ ] No publishing workflow was dispatched merely to test the rename. Its
+  end-to-end publishing test remains part of the separately approved pre-release.
+
+Reference: [GitHub's branch-renaming documentation](https://docs.github.com/en/repositories/configuring-branches-and-merges-in-your-repository/managing-branches-in-your-repository/renaming-a-branch).

README.md

@@ -40,6 +40,10 @@ For HPLuv (the pastel variant), use `hpluv_to_hex`, `hpluv_to_rgb`, `hex_to_hplu
 
 Run `python -m unittest discover -v`.
 
+## Repository maintenance
+
+See [BRANCH_MIGRATION.md](BRANCH_MIGRATION.md) for the planned default-branch rename to `main`.
+
 ## Authors
 
 * Robert McGinley ([mcginleyr1](https://github.com/mcginleyr1))