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-client/src/lib.rs

@@ -13,10 +13,19 @@
 //! also signed when the required wall-clock timestamps are available; negative
 //! values usually indicate client/server clock skew.
 //!
-//! [`Client`] is the runtime-free low-level blocking adapter. With the
-//! `tokio` feature, `AsyncClient` provides the corresponding Tokio adapter;
-//! [`managed`] contains the unified managed task/handle implementation,
-//! including the dedicated-runtime blocking frontend for synchronous callers.
+//! [`Client`] is the runtime-free low-level blocking adapter and requires no
+//! Tokio dependency at all; it is what a caller without a runtime, or that
+//! wants full control over its own event loop, should start from. With the
+//! `tokio` feature, `AsyncClient` provides the corresponding low-level Tokio
+//! adapter for a caller that owns a runtime and drives socket readiness
+//! itself. [`managed`] sits above both: `ManagedClientTask` /
+//! `ManagedClientHandle` (`tokio` feature) run and control one or more
+//! targets under a Tokio runtime, and `BlockingManagedClient` (`tokio`
+//! feature) wraps that in a synchronous owner backed by its own dedicated
+//! current-thread runtime for callers that want managed multi-target
+//! behavior without touching Tokio themselves.
+//!
+//! See `examples/` in the repository for a runnable example of each tier.
 //!
 #![forbid(unsafe_code)]