Query wrappers (distance/distances_from) + real-corpus benchmark; 0.3.0

[VladPr]

Summary

Closes two gaps from the post-0.2.0 list and preps the 0.3.0 release.

Query convenience wrappers (#4)

Adds the point-to-point and one-to-many query functions the README already advertised but never exported:

pub fn distance(cch: &CchView, metric: &MetricView, source: u32, target: u32) -> u32;
pub fn distances_from(cch: &CchView, metric: &MetricView, source: u32, targets: &[u32]) -> Vec<u32>;

Thin, zero-cost wrappers over the existing distance_matrix (single source), so they inherit its bit-identical, bounds-elided query path. Semantics match distance_matrix: INF_WEIGHT for unreachable, distance(s, s) == 0, distances_from(s, &[]) → empty vec.

Real-corpus benchmark (#3)

New env-gated benches/real_corpus.rs — measures customize parallel scaling (1 vs N threads) and a query suite on a genuine road network in the crate's own bundle format. Gated on CCH_CORPUS / CCH_CORPUS_METRIC; a plain cargo bench is a clean no-op (no cases registered). Nothing large is committed.

Measured on a real road network (Albania, ~6.29M nodes, 18 cores): customize runs 366.87 ms single-threaded → 80.7 ms on 18 threads (~4.55×) — notably better than the synthetic grids (~2.8× at 656k), consistent with real road networks having far better separators than grids. This is the honest continental data point the grid benches only approximated.

Release

Bumps 0.2.0 → 0.3.0 (minor: new public API), CHANGELOG [0.3.0] entry, README API table + real-corpus number + install pin.

Verification

• 190 lib + 35 equivalence + 13 load_struct + 3 smoke/doctest — all pass.
• clippy --all-targets -D warnings, fmt --check, cargo doc --no-deps (warning-free), and cargo llvm-cov --fail-under-lines 100 all clean.
• No new unsafe; distance_matrix, the query engine, Cch layout, and on-disk formats unchanged.

Cutting v0.3.0 after merge will be the first live exercise of the automated (Trusted Publishing) release pipeline.