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

AGENTS.md

@@ -52,7 +52,7 @@ Keep it stable. Prefer pointers to canonical sources over copied structure, file
 - Add only durable style, ownership, gotchas, and at most a few stable commands agents should actually run.
 - Prefer links or pointers to canonical sources over copied lists.
 - Avoid project trees, architecture maps, generated inventories, current file sizes, issue lists, TODO inventories, and other snapshots that will rot.
-- Stable commands: `cargo run -p hemx-xtask -- test`, `cargo run -p hemx-xtask -- html-examples-smoke`, `cargo check --workspace`, `redgate refs`. Use the xtask runner for full verification so jobs are capped from local CPU and memory and commands resolve the workspace independently of the caller's directory; use the html_examples smoke for focused repo-owned browser verification of the HTML pattern gallery, no-reload dynamic interactions, and no `/tmp` scripts. Keep fast crate tests, focused browser smoke, and full xtask authority distinct; the full path should stay within a documented 10 minute local timeout or be split into deterministic shards under the same wrapper. req: test/004 req: test/006 req: test/012 req: test/013 req: test/014 req: test/015 req: test/016
+- Stable commands: `cargo run -p hemx-xtask -- test`, `cargo run -p hemx-xtask -- mutation [PACKAGE]`, `cargo run -p hemx-xtask -- html-examples-smoke`, `cargo check --workspace`, `redgate refs`. Use the xtask runner for full verification so jobs are capped from local CPU and memory and commands resolve the workspace independently of the caller's directory; use the html_examples smoke for focused repo-owned browser verification of the HTML pattern gallery, no-reload dynamic interactions, and no `/tmp` scripts. Keep fast crate tests, focused browser smoke, and full xtask authority distinct; the full path should stay within a documented 10 minute local timeout or be split into deterministic shards under the same wrapper. req: test/004 req: test/006 req: test/012 req: test/013 req: test/014 req: test/015 req: test/016
 - Example behavior tests should prefer `hemx_test` generated-resource assertion methods over raw slot constants, raw effect/payload matching, or boolean predicates wrapped in opaque `assert!`; failures should include the expectation and actual effects, while rendered target/handle assertions should name the generated resource. Keep browser selector helpers as test adapters only, not authoring APIs. Process-backed tests use the RAII `TestProcess` harness rather than duplicating readiness loops and child cleanup. req: test/008 req: test/009 req: test/010 req: test/017 req: test/018 req: test/019
 - Run the workout product exemplar with `cargo run -p hemx-xtask -- workout dev` and open `http://127.0.0.1:3028`; set `HEMX_WORKOUT_ADDR=127.0.0.1:3030` if the default port is busy. Its durable visual direction and recovery expectations live in `examples/workout/DESIGN.md`. req: examples/008
 - Use the same Workout command surface for tests, production build, and mobile release: `cargo run -p hemx-xtask -- workout test`, `cargo run -p hemx-xtask -- workout build`, `HEMX_WORKOUT_ORIGIN=https://workout.example.com cargo run -p hemx-xtask -- workout mobile-release`, and `HEMX_WORKOUT_ORIGIN=https://workout.example.com cargo run -p hemx-xtask -- workout mobile-verify`; Android/iOS SDKs, store submission targets, and signing remain external blockers, not repo-owned secrets, and do not imply a broad `hemx-mobile` framework. req: examples/006 req: examples/011 req: examples/013