bench: add gcd benchmark case

[Sharjeelbaig]

Summary

• add a small gcd Zero benchmark case
• register the case in the benchmark harness
• document the new benchmark in the public benchmark docs

Closes #8

Validation

• make -C native/zero-c
• bin/zero check --json benchmarks/zero/gcd.0
• npm run docs:test
• bin/zero build --json --emit exe --target linux-musl-x64 benchmarks/zero/gcd.0 --out .zero/bench/zero-gcd-linux

ZERO_BENCH_RUNS=1 npm run bench was attempted locally on macOS arm64 but does not complete there: existing executable cases hit the Darwin dyld: missing LC_UUID load command issue tracked in #28. The new gcd case was also checked against the Linux direct executable backend, where it builds successfully.

[vercel]

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