chore(nix): update rust-overlay so Rust 1.97.1 resolves

[AlexHardAtWork]

What

Update the rust-overlay flake input (2026-06-01 → 2026-09-10).

Why

The committed flake.lock cannot build the dev shell. direnv allow / nix develop fails:

error: Stable 1.97.1 is not available

rust-toolchain.toml pins channel = "1.97.1", but the locked rust-overlay predates that
Rust release, so the overlay has no such toolchain to offer. Anyone setting up via the
documented "zero-friction path" hits this on first entry.

CI is unaffected — .github/workflows/ci.yml gets its toolchain from
dtolnay/rust-toolchain, which fetches from upstream and never consults the overlay. That
is why this went unnoticed.

Verification

nix develop evaluates again, with the pinned toolchain resolving to rust-minimal-1.97.1.

Note

This recurs on every rust-toolchain.toml bump: the pin has to exist in the locked overlay
as well. Worth remembering next to the existing comment in rust-toolchain.toml about
keeping the CI refs in sync.