Add committed profiling/benchmarks for the miner package

[JSONbored]

Problem: No profiling or benchmark data exists anywhere for the miner package — there's no baseline to know whether a future change makes discovery/claiming faster or slower.

Area: AMS / Performance

Proposal: Add a small, committed benchmark suite covering the discovery fanout and the local-store read/write paths.

Deliverables:

• A runnable benchmark script and a committed baseline result.

Acceptance criteria:

• The benchmark runs in CI or on demand and produces a comparable, repeatable number.

[linear-code]

JSO-3505