dev-deps: add a benchmark harness (divan or criterion) — there is no benches/ today

There is no benches/ directory and no benchmark harness in Cargo.toml. Several open dependency
questions (SIMD crates, packed-seq, external sort, sufr vs caps-sa, #162) all start with
"measure first", and each currently has to invent its own measurement. One harness, once, unblocks
all of them.

Candidates

Both are dev-dependencies only, so nothing ships in the binary and the supply-chain argument in
CONTRIBUTING.md is much weaker than for a runtime dep.

What to benchmark

The paths the open questions actually care about:

1. genomeGenerate suffix-array construction (src/index/sa_build.rs), in-mem and ext-mem arms
2. Seed extension / find_stop (src/align/simd_scan.rs)
3. FASTQ decode, plain and gzipped (src/io/fastq.rs)
4. BAM write and coordinate sort (src/io/bam.rs)
5. End-to-end align on a fixed small fixture, as a coarse regression signal

Constraints

• Benchmarks must not gate CI correctness; a slow machine must never fail the build.
• Numbers are machine-dependent. Useful as A/B within one machine and one session, not as absolute
  thresholds checked into tests.
• Keep the fixture small enough to run locally, large enough not to be dominated by process startup.

Checklist

• Pick one harness, not both
• Add the five benches above
• Document in CONTRIBUTING.md how to run them and how to report an A/B in a PR
• Link this issue from the dependency issues that are gated on measurement

Start by reading Cargo.toml and CONTRIBUTING.md, then inspect the listed paths: src/index/sa_build.rs, src/align/simd_scan.rs, src/io/fastq.rs, and src/io/bam.rs. Choose either divan or criterion, add the five requested benchmarks with a small fixed fixture, and document local execution and A/B reporting in CONTRIBUTING.md; done also includes linking the dependency issues.