feat: Gas benchmark regression CI gate

[raizo07]

closes #112

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[truthixify]

The stellar and stellar-bench-pr checks failed against an older workspace state that has since been fixed on develop. Rebase and force-push and CI should look different.

[truthixify]

Rebase against develop, conflicts appeared after the recent contracts merge cascade. Force-push and CI will re-fire.

[truthixify]

Two specific failures:

1. stellar fails on cargo fmt --all --check — run cargo fmt --all in stellar/ and commit.

2. stellar-bench-pr fails with error: unknown argument: --bench from your gas benchmark binary. Cargo passes --bench to the binary automatically after your other args (that's what the trailing --bench in the log means). Two options:

   • Make the gas binary silently accept/ignore an unknown --bench flag, OR
   • Switch the workflow invocation from cargo bench to cargo run --release --bin gas -- --format json --out bench/results.json (which won't append --bench).