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-app/README.md

@@ -0,0 +1,33 @@
+# irtt-rs
+
+The IRTT-compatible client, TUI, and server applications, packaged as Cargo
+package `irtt-rs`.
+
+Installing this crate builds:
+
+- `irtt-rs` — a multicall dispatcher binary that runs the client, TUI, or
+  server applet by subcommand.
+- `irtt-client` — the dedicated client binary (feature `client`).
+- `irtt-tui` — the interactive terminal UI (feature `tui`).
+- `irtt-server` — the dedicated server binary (feature `server`).
+
+All three features are enabled by default, so a plain `cargo install irtt-rs`
+installs everything.
+
+## This is an application, not a library
+
+This package's library target is undocumented (`doc = false`) and exists only
+to share code between its binaries. If you're looking for the client/session
+or server API to embed in your own Rust program, use
+[`irtt-client`](https://crates.io/crates/irtt-client) or
+[`irtt-server`](https://crates.io/crates/irtt-server) instead.
+
+## Usage
+
+See the [project repository](https://github.com/Lochnair/irtt-rs) for
+installation instructions, command-line usage, and configuration.
+
+## Project
+
+Part of [irtt-rs](https://github.com/Lochnair/irtt-rs), an independent Rust
+implementation of an IRTT-compatible protocol stack.