feat(xtask): add package-native mutation gate

Expose a capped mutation command that selects only elected packages, rejects unknown input, propagates mutest failures, and keeps output under the workspace target directory.

req: test/020

req: test/021

req: test/022

docs/v1-readiness.md

@@ -170,6 +170,7 @@ Run these on the final tree before GOAL_DONE:
 
 ```sh
 cargo run -p hemx-xtask -- test
+cargo run -p hemx-xtask -- mutation
 cargo check --workspace
 cargo test -p hemx-saas-example
 cargo test -p hemx-v0-examples