refactor(app): clarify application and binary roles

Rename the application Cargo package from irtt-cli to irtt-rs
(crates/irtt-cli -> crates/irtt-app), and hard-rename its scriptable
client binary from irtt-cli to irtt-client, now that the application
also ships a first-class server and the old name reads as an umbrella
CLI for both roles.

Give each dedicated binary (irtt-client, irtt-tui, irtt-server) its
own entry point instead of funneling through the multicall
dispatcher's argv0/subcommand logic, so a dedicated binary only links
the applet code for the role it implements and always runs that role
regardless of the name it's invoked or copied under. irtt-rs remains
the feature-selectable multicall dispatcher and is now the only binary
that inspects argv0 or subcommands.

Move future application releases to the irtt-rs/vX.Y.Z tag namespace
(regenerated via `dist generate`), and bump the application package to
0.6.0 as a breaking distribution change. Library crate versions are
unaffected.

dist-workspace.toml

@@ -11,7 +11,7 @@ ci = "github"
 installers = []
 # Target platforms to build apps for (Rust target-triple syntax)
 targets = ["aarch64-apple-darwin", "aarch64-unknown-linux-gnu", "x86_64-apple-darwin", "x86_64-unknown-linux-gnu", "x86_64-unknown-linux-musl", "x86_64-pc-windows-msvc"]
-# Scope dist's tag matching to irtt-cli releases only: irtt-proto/irtt-client/
+# Scope dist's tag matching to irtt-rs releases only: irtt-proto/irtt-client/
 # irtt-server/irtt-stats are libraries published to crates.io and tagged for
 # provenance, but must not trigger cargo-dist or produce a GitHub Release.
-tag-namespace = "irtt-cli"
+tag-namespace = "irtt-rs"