Clippy clean, and rustfmt and clippy part of the checks

cargo clippy --workspace --all-targets had 24 warnings; its own fixes took
most, the rest by hand: two type aliases for long types, a struct built
whole, a checked division, a doc comment's stray blank line. Its fix for
the stale-feed fold was wrong: `any` stops at the first feed it removes,
where the fold removed every one, so that is a loop instead.

A pre-commit hook in .githooks refuses unformatted Rust; clippy, being a
build, is in CLAUDE.md's checks with -D warnings.

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

.git-blame-ignore-revs

@@ -0,0 +1,3 @@
+# Formatting only: `git config blame.ignoreRevsFile .git-blame-ignore-revs` makes blame skip these.
+# rustfmt adopted
+692a5102179b958b5d15cecc4f5e6f16b2955b76