docs(crates): give each published crate its own crates.io README and examples

Each published crate (irtt-rs, irtt-client, irtt-server, irtt-stats,
irtt-proto) now points crates.io at its own README instead of
inheriting the root project README via the workspace-wide readme
default, which is removed. Each crate README is a short landing page:
what the crate is, who should use it, which sibling crate to use
instead if this isn't the right abstraction level, and links to
docs.rs and the main repository.

Add real Cargo example targets teaching irtt-client's blocking/Tokio/
managed API tiers, a basic and a multi-listener irtt-server example,
and an irtt-stats example contrasting finite vs continuous retention.
irtt-proto intentionally has none. Update each crate's top-level
rustdoc to explain its API-tier choices and point at the examples, and
enable docs.rs rustdoc example scraping for irtt-client and
irtt-server (via doc-scrape-examples on each example target, needed
because the crate has dev-dependencies).

The client examples are caller-paced against next_send_deadline()
rather than sending on every loop iteration, bound their receive wait
by the next send/timeout deadline instead of awaiting indefinitely,
poll timeouts on a short fixed cadence, and surface send/receive/close
errors instead of discarding them via unwrap_or_default()/if-let-Ok --
so they demonstrate correct pacing and terminate/report cleanly rather
than racing the server, hanging on a lost reply, or silently dropping a
failed final close.

crates/irtt-stats/README.md

@@ -0,0 +1,38 @@
+# irtt-stats
+
+Statistics aggregation over [`irtt-client`](https://crates.io/crates/irtt-client)
+events: loss/duplicate/late accounting, RTT and one-way-delay timing, and
+inter-packet delay variation (IPDV), as cumulative and rolling snapshots.
+
+## Relationship to irtt-client
+
+This crate does not open sessions or drive sockets. Feed it the `ClientEvent`
+stream produced by an `irtt-client` `Client`, `AsyncClient`, or managed
+session via `StatsCollector::process`, and read back a `Snapshot`.
+
+A `StatsCollector` has one sequence/IPDV namespace and one packet count, both
+scoped to a single target. A managed session covering multiple targets needs
+one collector per target — as the `irtt-rs` CLI does — since a target's
+sequence numbers start over at zero and feeding two targets into one
+collector would pair up unrelated probes.
+
+## Retention modes
+
+`StatsConfig::finite()` retains exact timing samples for its cumulative
+snapshot, so every metric with samples reports an exact median there —
+retention grows with the probe count. `StatsConfig::continuous()` keeps
+bounded running statistics instead (no exact median) plus a bounded
+4096-entry adjacent-sequence IPDV store per target, for long-running or
+unbounded sessions. Rolling-window snapshots are always running-only and
+report no medians, whichever `StatsConfig` produced them.
+
+See `examples/` in the repository for a runnable comparison of both modes.
+
+## Documentation
+
+Full API documentation is on [docs.rs/irtt-stats](https://docs.rs/irtt-stats).
+
+## Project
+
+Part of [irtt-rs](https://github.com/Lochnair/irtt-rs), an independent Rust
+implementation of an IRTT-compatible protocol stack.