Add benchmark harness and compiled evaluator

[chboishabba]

This PR adds a deterministic benchmark entrypoint and a compiled evaluator path for FRACTRAN programs.

Changes:

• add fractran-bench CLI via src/Bench.hs and src/bench_main.hs
• add compiled exponent-vector evaluator in src/Compiled.hs
• update build.sh to build both fractran and fractran-bench
• default to -dynamic builds, while --profile keeps the profiling build path
• document benchmark usage in README.md
• ignore benchmark binary and profiling output in .gitignore

Notes:

• existing demo flow remains available via ./fractran
• benchmark engines include naive-fast, reg, frac-opt, cycle, compiled, and lut
• scenarios currently include mult_smoke, primegame_small, primegame_medium, and primegame_large
• the branch is kept to one commit for review

If you want a shorter version, use:

Adds a benchmark harness and a compiled exponent-vector evaluator.

Summary:

• new fractran-bench CLI
• new Compiled.hs evaluator path
• build.sh now builds both fractran and fractran-bench
• README updated with benchmark usage
• .gitignore updated for benchmark/profiling artifacts

This keeps the existing demo binary intact and packages the benchmark work as a single reviewable commit.

This came out of downstream benchmarking work in FRACDASH, but the change set here is kept upstream-focused and self-contained.