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/src/lib.rs

@@ -1,7 +1,24 @@
 //! Statistics aggregation for `irtt-client` events.
 //!
-//! The crate consumes `irtt-client` events and produces cumulative or rolling
-//! snapshots for reporting and integration code.
+//! [`StatsCollector`] consumes [`irtt_client::ClientEvent`] values through
+//! [`StatsCollector::process`] and produces a cumulative [`Snapshot`], plus
+//! optional count- and time-based rolling snapshots, for reporting and
+//! integration code.
+//!
+//! # Retention modes
+//!
+//! [`StatsConfig`] selects between two retention modes via [`SampleMode`].
+//! [`StatsConfig::finite`] ([`SampleMode::Exact`]) retains an exact sample per
+//! timing metric, so every metric reports an exact median once it has
+//! samples; retention grows with the number of probes processed. This is the
+//! default and suits a run with a known, bounded duration.
+//! [`StatsConfig::continuous`] ([`SampleMode::RunningOnly`]) keeps only
+//! running statistics — no exact median — and bounds its adjacent-sequence
+//! IPDV tracking, so memory stays bounded for a long-running or unbounded
+//! session. Rolling snapshots always use running statistics regardless of
+//! this setting.
+//!
+//! See `examples/` in the repository for a runnable comparison of both modes.
 
 #![forbid(unsafe_code)]
 #![warn(missing_docs)]