Format the Rust code with rustfmt

The code was formatted by hand, and `cargo fmt --check` differed in 866
places. rustfmt.toml keeps the house's long lines (120) and short
expressions on one line, which halves what changes; this commit is only
what rustfmt does with it, and is named in .git-blame-ignore-revs.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

rustfmt.toml

@@ -0,0 +1,4 @@
+# Matches how the code was written by hand before rustfmt was adopted: long lines, and short
+# expressions kept on one line.
+max_width = 120
+use_small_heuristics = "Max"