chore: ignore the style-edition reformat in git blame

The style-edition reformat touched 69 files without changing behaviour,
which buries the real authorship of those lines. GitHub's web blame applies
.git-blame-ignore-revs automatically; locally it needs blame.ignoreRevsFile
set, so CONTRIBUTING now says so where contributors first clone.

Only the reformat is listed. The let-chain commit also touches many files
but changes control flow and drops a crate-wide lint allow, so its
authorship is real and should stay visible in blame.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

.git-blame-ignore-revs

@@ -0,0 +1,10 @@
+# Bulk-formatting commits, ignored so `git blame` reports the author who wrote
+# a line rather than whoever last reformatted it. GitHub's web blame applies
+# this automatically; locally it needs opting in once:
+#
+#   git config blame.ignoreRevsFile .git-blame-ignore-revs
+#
+# Only commits with no behaviour change belong here.
+
+# chore: reformat under rustfmt style edition 2024
+ad63139576f77767299ac1c0f4bca5b52fe80442

CONTRIBUTING.md

@@ -189,6 +189,13 @@ git pull origin develop
 git checkout -b feat/scope-your-clear-description
 ```
 
+Opt in once per clone so `git blame` skips the bulk-formatting commits listed in
+`.git-blame-ignore-revs` and reports whoever actually wrote each line:
+
+```bash
+git config blame.ignoreRevsFile .git-blame-ignore-revs
+```
+
 ### 2. Make Your Changes
 
 **Respect the existing folder structure.** Place new files where similar files already live. Do not reorganize without prior discussion.