Add solbench methods: per-method read-latency matrix (roadmap 6)

[jenish-25]

Summary

Ships the first piece of roadmap item 6: a per-method read-latency matrix via a new
solbench methods subcommand. Today probe measures one hardcoded method (getSlot); this
measures a set of read methods across every endpoint and reports a latency distribution per
(method × endpoint).

Motivation: different RPC methods carry different server-side cost, so a matrix lets you compare
endpoints within a method and see relative method cost — an honestly-labeled distribution
metric, which CONTRIBUTING.md calls out as especially welcome.

What's included

• solbench methods --samples N --interval-ms M [--account <pubkey>] [--json] — measures
  getSlot, getVersion, getLatestBlockhash, getAccountInfo, getMultipleAccounts.
• Reuses the fair sampler. The open-loop, coordinated-omission-correcting tick loop is
  extracted from probe.rs::run_endpoint into a shared sample_endpoint(...), so probe and
  methods measure identically. getSlot/slot-lag behavior is preserved exactly (a thin
  extract closure), and probe_all/ProbeResult are unchanged.
• Provider-neutral, reproducible params. The account-reading methods hit canonical Solana
  sysvars (Clock / Rent / RecentBlockhashes), which exist on any cluster; --account overrides the
  getAccountInfo target.
• Honest labeling. Output footer states the numbers are network-inclusive round-trips from THIS
  host, to compare endpoints within a method (not across), that methods run in sequence, and
  which fixed accounts are used.
• solbench-core is untouched — no new math there, so the "keep core network-free" rule holds.

Example

method               endpoint         host                          p50ms    p99ms   jitter    ok
getSlot              public mainnet   api.mainnet-beta.solana.com   663.34   765.15    51.40   3/3
getVersion           public mainnet   api.mainnet-beta.solana.com   766.48   880.46    84.40   3/3
getLatestBlockhash   public mainnet   api.mainnet-beta.solana.com   786.44  1838.29   500.97   3/3
getAccountInfo       public mainnet   api.mainnet-beta.solana.com   770.34   811.45    19.83   3/3
getMultipleAccounts  public mainnet   api.mainnet-beta.solana.com   655.33   752.42    45.92   3/3

--json emits [{ "method", "results": [{ "label", "host", "ok", "errors", "latency" }] }].

Testing

cargo fmt --all --check, cargo clippy --all-targets --all-features -- -D warnings, and
cargo test --all all pass locally. New tests use a hermetic tiny_http server (no live network):

• sample_endpoint records latency + extracts values, and counts malformed responses as errors;
• probe_methods produces a per-method report with latency;
• method_specs are well-formed JSON with the right method/params;
• methods_extract distinguishes result vs error.

Notes

• Public endpoints may rate-limit under high --samples across five methods; defaults are modest.
• Scope is intentionally the matrix only; HDR histograms (also under roadmap 6) are left as a
  separate change. Happy to adjust the method set, output columns, or naming to your preferences.