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.

docs/INTEROP_COMPARISON.md

@@ -13,7 +13,7 @@ upstream executable.
 Build `irtt-rs` and ensure an upstream `irtt` executable is available:
 
 ```sh
-cargo build -p irtt-cli
+cargo build -p irtt-rs --bin irtt-client
 go install github.com/heistp/irtt/cmd/irtt@v0.9.1
 ```
 
@@ -118,13 +118,13 @@ python3 scripts/run_netem_interop_ci.py \
 On Linux:
 
 ```sh
-cargo build -p irtt-cli
+cargo build -p irtt-rs --bin irtt-client
 go install github.com/heistp/irtt/cmd/irtt@v0.9.1
 python3 scripts/run_netem_interop_ci.py \
   --all \
   --skip-build \
   --upstream-irtt "$HOME/go/bin/irtt" \
-  --irtt-rs-command "$PWD/target/debug/irtt-cli"
+  --irtt-rs-command "$PWD/target/debug/irtt-client"
 ```
 
 Run one scenario: