ADR-0022 stage 3/8: simple-mode echo lines highlighted

Lift `dsl::ECHO_PREFIX = "running: "` as a public const,
with a unit test asserting `t!("dsl.running", input = "")`
matches it. The catalog template is now contracted to equal
`format!("{ECHO_PREFIX}{input}")` — a translator changing
the prefix breaks the test.

Add `input_render::lex_to_runs(input, theme)` — a
cursor-less variant of `render_input_runs` for use cases
(echo lines, future hint panel) that need token-class
colouring without an inverted cursor.

ui::render_output_line: when the line is an Echo submitted
in Simple mode, peel the prefix and re-tokenise the rest
through lex_to_runs, rendering each token at its class
colour. Advanced-mode echoes and any echo whose body
unexpectedly lacks the prefix fall through to the plain
rendering.

Tests: 683 passing, 0 failing, 1 ignored (682 baseline →
+1 echo_prefix_matches_catalog_template). Clippy clean
(uses let-chain to keep the if condition flat).

Stage 4 adds render-time parse + error overlay so the
failing token in mid-typed input lights up in the error
colour.

src/dsl/mod.rs

@@ -26,3 +26,31 @@ pub use command::{
 pub use parser::{ParseError, parse_command};
 pub use types::Type;
 pub use value::Value;
+
+/// Prefix every echoed DSL command carries in the output
+/// panel — i.e. `[simple] running: <input>` reads as
+/// `[simple]` (tag) + `running: ` (this constant) + the
+/// user's input.
+///
+/// The catalog template `dsl.running` is contracted to equal
+/// `format!("{ECHO_PREFIX}{{input}}")`. The constant lives
+/// in code because the echo-line renderer (ADR-0022 §5)
+/// peels this prefix off and re-tokenises the rest for
+/// highlighting; a unit test (`echo_prefix_matches_catalog_template`)
+/// pins the binding.
+pub const ECHO_PREFIX: &str = "running: ";
+
+#[cfg(test)]
+mod tests {
+    use super::ECHO_PREFIX;
+
+    #[test]
+    fn echo_prefix_matches_catalog_template() {
+        // The catalog template `dsl.running` must produce
+        // `<ECHO_PREFIX><input>` so the echo-line renderer can
+        // peel the prefix and re-tokenise the rest. A
+        // translator changing the prefix breaks this test.
+        let rendered = crate::t!("dsl.running", input = "");
+        assert_eq!(rendered, ECHO_PREFIX);
+    }
+}