fix(cli): aura run refuses an open blueprint with a clean exit 2, not a panic

`aura run <blueprint.json>` bootstraps the loaded signal over the EMPTY
param point (a closed-blueprint assumption) and `.expect()`s
`compile_with_params`, which panics (exit 101, ParamArity) when the
blueprint carries >=1 free knob. The sibling `aura mc` already refuses an
open blueprint cleanly at the dispatch boundary; `run` now does the same.

The `["run", ..]` `.json`-discriminator arm probes the wrapped param_space
(`blueprint_axis_probe`, the single source of that wrap) and, when it is
non-empty, emits a "run requires a closed blueprint … N free knob(s) —
bind them or use `aura sweep --axis`" diagnostic and exits 2 — mirroring
`blueprint_mc_family`'s closed-guard. No panic on a naive consumer's first
move (author an open blueprint, run it).

RED-first: cli_run.rs `aura_run_rejects_an_open_blueprint_without_panicking`
pins exit 2 + the named requirement + no `panicked`. Surfaced by the
World/C21 milestone fieldtest.

closes #176
refs #170

crates/aura-cli/src/main.rs

@@ -4256,6 +4256,18 @@ fn main() {
                     eprintln!("aura: {path}: {e:?}");
                     std::process::exit(2);
                 });
+                // Refuse an open (free-knob) blueprint at the dispatch boundary, mirroring
+                // `blueprint_mc_family`'s closed-guard: `run` bootstraps over the EMPTY point, so a
+                // free knob would panic in `compile_with_params` — reject it clean instead (#176).
+                let free = blueprint_axis_probe(&doc).param_space();
+                if !free.is_empty() {
+                    eprintln!(
+                        "aura: run requires a closed blueprint (no free parameters); {} free knob(s) — \
+                         bind them or use `aura sweep --axis`",
+                        free.len()
+                    );
+                    std::process::exit(2);
+                }
                 let BlueprintRunArgs { params, data, seed } = parse_blueprint_run_args(&rest[1..])
                     .unwrap_or_else(|msg| {
                         eprintln!("aura: {msg}");