docs(crate): ENG-4683 README, axum quickstart, and architecture diagram

.gitignore

@@ -11,6 +11,13 @@
 # The downstream-consumer test sub-package is a library crate too; same
 # rationale for excluding its lockfile.
 tests/downstream_consumer/Cargo.lock
+# `/target/` above is root-anchored, so it does not cover the nested
+# sub-package. Running the sub-package's suite directly — e.g.
+# `cargo test --manifest-path tests/downstream_consumer/Cargo.toml`,
+# which is how you reproduce ENG-4683's AC3 locally — drops ~300 MB of
+# build output here. tests/downstream_consumer_smoke.rs itself redirects
+# to a target dir under $OUT_DIR and never creates this.
+tests/downstream_consumer/target/
 
 # macOS — https://github.com/github/gitignore/blob/main/Global/macOS.gitignore
 .DS_Store

CHANGELOG.md

@@ -19,6 +19,15 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   `cargo codspeed`). `[skip-perf]` in a PR title bypasses the perf job.
   Uploads authenticate to CodSpeed over OpenID Connect (`id-token: write`)
   via the org's CodSpeed GitHub App — no `CODSPEED_TOKEN` secret required.
+- Onboarding documentation (ENG-4683): a full `README.md` with a
+  copy-pasteable library quickstart, a 50-line axum HTTP server
+  quickstart, the build-pipeline diagram, the `data-{N}km` feature
+  matrix, and the 13 edge groups. The axum snippet ships as a runnable
+  `examples/axum_server.rs`, and both quickstarts work on default
+  features (`data-50km`) with no data setup. Drift is pinned by tests:
+  README fences compile under `cargo test --doc`, the edge-group table
+  is compared against `EDGE_GROUPS`, and an end-to-end test boots the
+  example and asserts GeoJSON `[lng, lat]` output.
 - Automated release pipeline (ENG-4692): `release-plz` opens a
   `chore: release vX.Y.Z` PR from conventional commits and, on merge,
   publishes to crates.io and creates the GitHub release/tag;

Cargo.toml

@@ -71,12 +71,27 @@ tempfile = "3"
 # deny.toml's licence allowlist — no library or downstream-consumer impact.
 serde = { version = "1", features = ["derive"] }
 serde_json = "1"
+# ENG-4683: examples/axum_server.rs (the README's HTTP quickstart).
+# Dev-only — no library or downstream-consumer impact, and the wasm CI
+# job builds lib-only (`cargo build`, no --all-targets) so these never
+# cross-compile. Both MUST stay on ONE line: tests/changelog.rs:32-43
+# derives the crate version from the first `version`-prefixed line, so a
+# multi-line inline table here would introduce a second candidate.
+# `net` is beyond the feature list ENG-4683 originally named: axum::serve
+# takes a tokio::net::TcpListener, which that feature gates. axum's own
+# `tokio` feature would activate it transitively, but the example names
+# the path directly, so we declare it directly.
+axum = "0.8"
+tokio = { version = "1", features = ["macros", "net", "rt-multi-thread"] }
 # ENG-4690: CodSpeed's drop-in criterion replacement. Behaves as plain
 # criterion under `cargo bench` (incl. html_reports) and registers benches
 # with the CodSpeed harness under `cargo codspeed`. Re-exports the full
 # criterion 0.5 API, so benches/route.rs is ordinary criterion code.
 codspeed-criterion-compat = { version = "2", features = ["html_reports"] }
 
+# NOTE: every [dev-dependencies] entry must stay ABOVE this table —
+# [[bench]] starts a new table, so anything added below it would silently
+# become part of the bench target instead of a dependency.
 [[bench]]
 name = "route"
 harness = false