Blame skips the sweep and answers with the author

The commit before this one rewrote a hundred and fifty-three files without
changing what any of them does. Left alone, that is what `git blame` would name
for half the lines in the repo — the formatter, standing in front of whoever
actually wrote the line and whatever they were thinking at the time.

`.git-blame-ignore-revs` is the file that fixes it. GitHub reads it with no setup
at all, so the blame view on the web is already right; locally it needs
`git config blame.ignoreRevsFile .git-blame-ignore-revs` once, which is now step
two of the dev install in CONTRIBUTING alongside the editable install, since a
contributor who never runs it gets the wrong answer silently rather than an
error.

The file's header states the bar for being in it: whole-tree, provably
behaviour-free. That is a deliberately narrow door — an entry here makes a commit
invisible to the tool people use to ask why a line looks the way it does, and a
commit that changed anything real must never be able to hide behind it.

Verified rather than assumed: `git blame` on a line the sweep rewrote now reports
`f07f8c1b` from July, which is the commit that actually wrote it.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

.git-blame-ignore-revs

@@ -0,0 +1,13 @@
+# Commits that changed how the code is spelled, never what it does. `git blame` skips
+# them so a line's history points at whoever last had something to say about it.
+#
+# GitHub reads this file by itself. Locally it has to be wired up once:
+#
+#     git config blame.ignoreRevsFile .git-blame-ignore-revs
+#
+# Only whole-tree, provably behaviour-free commits belong here.
+
+# Whole-repo `ruff format .` before the public release: 153 files, 152 of them verified
+# AST-identical, the suite green at 1888, and `meshterm specimen` byte-identical on both
+# platforms.
+92d31a31913b036a76ed27c20863f65a53b0e14d

CONTRIBUTING.md

@@ -34,6 +34,7 @@ this repository.
 2. Set up a dev install:
    ```bash
    pip install -e ".[dev]"
+   git config blame.ignoreRevsFile .git-blame-ignore-revs   # skip the formatting sweeps
    ```
 3. Before opening the PR, make sure these all pass:
    ```bash