feat(yahweh): MockBackend público + tests E2E del widget con TestAppContext

Cierra el ciclo de testabilidad del widget metainterfaz. Hasta
ahora los tests del MetaBackend trait vivían como impl privada en
backend.rs; el widget no podía testear handlers sin levantar
NakuiBackend (que depende de event log + Rhai).

yahweh-meta-runtime:
- Nuevo `pub mod testing` con MockBackend (renombre del MemBackend
  privado, ahora público). Constructores: new(), with_records(iter),
  with_morphism(name, handler) builder. Métodos de inspección
  total_records / records_for. Bajo `pub mod testing` (no cfg test)
  para que crates downstream lo usen en sus dev tests.
- Tests del trait en backend.rs simplificados: usan MockBackend en
  vez del MemBackend duplicado. 8 backend.rs + 9 nuevos del mock.

yahweh-widget-meta-form:
- Dev-dep nueva: gpui con feature "test-support" (TestAppContext).
- MetaApp::apply_action ahora pub (era privado). Necesario para
  invocar handlers desde tests E2E.
- Nuevo tests/widget_with_mock_backend.rs con 4 tests #[gpui::test]:
  meta_app_constructs, open_view_action_does_not_panic,
  backend_state_visible_from_widget_perspective,
  morphism_handler_can_be_registered_and_called_via_widget.

Tests: 47→56 yahweh-meta-runtime, 3→7 yahweh-widget-meta-form.
Total stack 109 verdes.

Limitación: render() no se invoca (requiere window context más
rico). Tests verifican state machine, no pixels. Snapshot tests
serían scope futuro.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Cargo.lock

@@ -17,6 +17,15 @@ dependencies = [
  "psl-types",
 ]
 
+[[package]]
+name = "addr2line"
+version = "0.25.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "1b5d307320b3181d6d7954e663bd7c774a838b8220fe0593c86d9fb09f498b4b"
+dependencies = [
+ "gimli",
+]
+
 [[package]]
 name = "adler2"
 version = "2.0.1"
@@ -861,6 +870,21 @@ dependencies = [
  "arrayvec 0.7.6",
 ]
 
+[[package]]
+name = "backtrace"
+version = "0.3.76"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "bb531853791a215d7c62a30daf0dde835f381ab5de4589cfe7c649d2cbe92bd6"
+dependencies = [
+ "addr2line",
+ "cfg-if",
+ "libc",
+ "miniz_oxide",
+ "object",
+ "rustc-demangle",
+ "windows-link 0.2.1",
+]
+
 [[package]]
 name = "base-x"
 version = "0.2.11"
@@ -3840,6 +3864,25 @@ dependencies = [
  "weezl",
 ]
 
+[[package]]
+name = "gimli"
+version = "0.32.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "e629b9b98ef3dd8afe6ca2bd0f89306cec16d43d907889945bc5d6687f2f13c7"
+
+[[package]]
+name = "git2"
+version = "0.20.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "7b88256088d75a56f8ecfa070513a775dd9107f6530ef14919dac831af9cfe2b"
+dependencies = [
+ "bitflags 2.11.1",
+ "libc",
+ "libgit2-sys",
+ "log",
+ "url",
+]
+
 [[package]]
 name = "glob"
 version = "0.3.3"
@@ -3923,6 +3966,7 @@ dependencies = [
  "as-raw-xcb-connection",
  "ashpd 0.11.1",
  "async-task",
+ "backtrace",
  "bindgen",
  "blade-graphics",
  "blade-macros",
@@ -4139,12 +4183,15 @@ dependencies = [
  "dunce",
  "futures",
  "futures-lite 1.13.0",
+ "git2",
  "globset",
  "gpui_collections",
+ "gpui_util_macros",
  "itertools 0.14.0",
  "libc",
  "log",
  "nix 0.29.0",
+ "rand 0.9.4",
  "regex",
  "rust-embed",
  "schemars 1.2.1",
@@ -5286,6 +5333,18 @@ dependencies = [
  "cc",
 ]
 
+[[package]]
+name = "libgit2-sys"
+version = "0.18.4+1.9.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "9b26f66f35e1871b22efcf7191564123d2a446ca0538cde63c23adfefa9b15b7"
+dependencies = [
+ "cc",
+ "libc",
+ "libz-sys",
+ "pkg-config",
+]
+
 [[package]]
 name = "libloading"
 version = "0.8.9"
@@ -5679,6 +5738,18 @@ dependencies = [
  "vcpkg",
 ]
 
+[[package]]
+name = "libz-sys"
+version = "1.1.28"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "fc3a226e576f50782b3305c5ccf458698f92798987f551c6a02efe8276721e22"
+dependencies = [
+ "cc",
+ "libc",
+ "pkg-config",
+ "vcpkg",
+]
+
 [[package]]
 name = "linfa-linalg"
 version = "0.1.0"
@@ -6388,6 +6459,7 @@ dependencies = [
  "serde_json",
  "tempfile",
  "uuid",
+ "yahweh-meta-runtime",
 ]
 
 [[package]]
@@ -8839,6 +8911,12 @@ dependencies = [
  "wasm-bindgen",
 ]
 
+[[package]]
+name = "rustc-demangle"
+version = "0.1.27"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "b50b8869d9fc858ce7266cce0194bd74df58b9d0e3f6df3a9fc8eb470d95c09d"
+
 [[package]]
 name = "rustc-hash"
 version = "1.1.0"