Fix formatting drift after rename, and document git-init requirement

Found via an end-to-end test of nix flake init -> rename -> nix flake
check: renaming can reorder `use` statements alphabetically (e.g.
"acme_widget" now sorts before "criterion"), and treefmt's rustfmt
(nixpkgs) disagreed with cargo fmt's rustfmt (fenix toolchain) on
edition-2024 import grouping, so the formatting check failed right
after a clean rename.

- nix/treefmt.nix: pin programs.rustfmt.edition = "2024" to match
  Cargo.toml, so treefmt and cargo fmt agree.
- scripts/rename-project.sh: auto-format (cargo fmt, falling back to
  nix fmt) after renaming, and note that nix commands need git-tracked
  files.
- README.md/CLAUDE.md: document that `nix flake init -t` doesn't
  git-init for you, and every nix command needs `git add` first.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018ALEXkc7pTro7tF1WcUuKb

scripts/rename-project.sh

@@ -48,7 +48,22 @@ else
   echo "warning: cargo not on PATH — run 'cargo generate-lockfile' yourself (e.g. inside 'nix develop')" >&2
 fi
 
+# Renaming can reorder `use` statements alphabetically (e.g. "acme_widget"
+# now sorts before "criterion"), which rustfmt/treefmt will otherwise flag
+# as unformatted on the next check. Reformat immediately so the tree is
+# clean right after renaming.
+if command -v cargo >/dev/null 2>&1 && cargo fmt --version >/dev/null 2>&1; then
+  cargo fmt
+elif command -v nix >/dev/null 2>&1 && git rev-parse --is-inside-work-tree >/dev/null 2>&1; then
+  nix fmt >/dev/null 2>&1 || echo "warning: 'nix fmt' failed — run it yourself once files are committed" >&2
+else
+  echo "warning: could not auto-format — run 'cargo fmt' or 'nix fmt' yourself" >&2
+fi
+
 echo "Renamed '${old_name}' -> '${new_name}'."
 echo
 echo "Review the diff (git diff), then optionally rename the project directory:"
 echo "  cd .. && mv $(basename "$root") ${new_name}"
+echo
+echo "If this tree isn't committed to git yet, run 'git add -A' before any"
+echo "'nix' command — Nix flakes only see git-tracked files."