fix(ci): make the Rust pipeline deterministic and green

[keanji-x]

Problem

Rust CI is red on main and on PRs (e.g. #50). The Check job fails at the
fmt step, which blocks Build (needs: check).

The failure was confusing because cargo fmt --all -- --check passes locally
but fails in CI. Root cause: gravity_bench ships no rustfmt.toml of its
own.

• In CI it is checked out standalone, so rustfmt falls back to its
  defaults → 91 files are reported non-compliant → fmt fails.
• Locally it lives nested under the gravity-sdk monorepo, so rustfmt
  walks up and silently uses the parent gravity-sdk/rustfmt.toml
  (use_small_heuristics = "Max", …) → the same tree looks clean.

Two more problems were latent behind the fmt failure (they only surface once
fmt is unblocked, so CI never reached them):

• The toolchain floats: the workflow uses dtolnay/rust-toolchain@stable
  while the repo pins 1.91.0 via rust-toolchain.toml — non-deterministic
  (a new stable release silently changes formatter/lints).
• Three tests would hang cargo test --all-features in CI: two
  #[tokio::test]s require a live node at localhost:8545, and
  test_find_account_by_address brute-forces 1,000,000 key derivations
  (observed running >60 s) for a target address not in range.

Fix

Change	Why
add rustfmt.toml (use_small_heuristics = "Max", reorder_imports, use_field_init_shorthand)	Make formatting self-contained — identical standalone (CI) or nested (local). The tree is already compliant, so this is config-only, no reformatting.
pin toolchain to 1.95.0 in rust-toolchain.toml and the workflow (dtolnay/rust-toolchain@1.95.0, both jobs)	Latest stable, deterministic — no more floating @stable.
#[ignore] the 2 node-dependent tests + the 1M-key brute-force lookup	They need infra / are manual utilities; run on demand with cargo test -- --ignored.

Verification (locally, toolchain 1.95.0)

cargo fmt --all -- --check     # clean (0 diffs, 0 warnings)
cargo build --release --all-features   # Finished
cargo test --all-features      # test result: ok. 2 passed; 0 failed; 3 ignored

cargo check --all-targets --all-features also passes — so all four CI steps
(fmt → check → test → build) are green.

Note: dtolnay/rust-toolchain@1.95.0 is a valid ref (the action publishes a
1.95.0 branch). The node-dependent / brute-force tests still run on demand
via cargo test -- --ignored.

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