mlx, mlxrunner: move the MLX engine out of x/

The MLX runner is the only Go inference runner left and is no longer
experimental, so its packages leave x/. The bindings become a top-level
mlx package beside the carried patches in mlx/compat, mirroring how
llama/ holds the llama.cpp integration, and the runner becomes mlxrunner
with the architectures nested under the package they implement.
Subpackages move with their parent unless listed.

  x/mlxrunner/mlx            mlx
  x/internal/mlxthread       mlx/mlxthread
  x/internal/mlxthreadtest   mlx/mlxthread/mlxthreadtest
  x/internal/mlxtest         mlx/mlxtest
  x/quant                    mlx/quant
  mlx/compat/*.patch         mlx/compat/mlx-c   (MLX patches go in mlx/compat/mlx)
  x/mlxrunner                mlxrunner
  x/models/nn                mlxrunner/nn
  x/models/<arch>            mlxrunner/model/<arch>
  x/mlxrunner/imports.go     mlxrunner/model/architectures   (new package)
  x/create                   create
  x/safetensors              fs/safetensors
  x/tokenizer                mlxrunner/tokenizer

Every package keeps its name, so the Go changes are the import path
rewrites the moves force, and the CMake, Dockerfile, CI cache keys, drift
check and Darwin payload script follow the new paths. Four edits are not
paths: the runner's blank architecture imports become the package
mlxrunner/model/architectures, so the list to extend for a new model sits
beside the architecture directories; a depguard rule keeps the two test
harnesses out of non-test code, as the x/internal placement used to; the
CI change filter's two entries for the long-deleted x/imagegen/mlx now
name the bindings' CMake project and the carried patches, so a change to
either builds the payload; and the tokenizer parity test reads its
fixtures from its own testdata instead of walking out of x/.

x/server and x/imagegen/manifest stay for the next two commits.

.github/workflows/test.yaml

@@ -49,10 +49,11 @@ jobs:
             'LLAMA_CPP_VERSION' \
             'MLX_VERSION' \
             'MLX_C_VERSION' \
-            'x/imagegen/mlx/**' \
-            'x/imagegen/mlx/**/*' \
-            'x/mlxrunner/xgrammar/native/**' \
-            'x/mlxrunner/xgrammar/native/**/*' \
+            'mlx/CMakeLists.txt' \
+            'mlx/compat/**' \
+            'mlx/compat/**/*' \
+            'mlxrunner/xgrammar/native/**' \
+            'mlxrunner/xgrammar/native/**/*' \
             '.github/**/*') | tee -a $GITHUB_OUTPUT
           echo app_changed=$(changed 'app/**' 'app/**/*') | tee -a $GITHUB_OUTPUT
           echo go_mod_changed=$(changed 'go.mod') | tee -a $GITHUB_OUTPUT
@@ -417,7 +418,7 @@ jobs:
         with:
           path: .cache/mlx-darwin-release
           # Key on every payload input so a source-built payload survives pushes.
-          key: mlx-darwin-${{ hashFiles('MLX_VERSION', 'MLX_C_VERSION', 'cmake/local.cmake', 'cmake/apply-git-patches.cmake', 'cmake/mlx/CMakeLists.txt', 'cmake/mlx/CMakePresets.json', 'x/mlxrunner/mlx/CMakeLists.txt', 'mlx/compat/**', 'x/mlxrunner/xgrammar/native/**') }}
+          key: mlx-darwin-${{ hashFiles('MLX_VERSION', 'MLX_C_VERSION', 'cmake/local.cmake', 'cmake/apply-git-patches.cmake', 'cmake/mlx/CMakeLists.txt', 'cmake/mlx/CMakePresets.json', 'mlx/CMakeLists.txt', 'mlx/compat/**', 'mlxrunner/xgrammar/native/**') }}
       - name: Prepare MLX Darwin release payload
         if: ${{ startsWith(matrix.os, 'macos') }}
         run: .github/scripts/prepare_mlx_darwin.sh
@@ -437,11 +438,9 @@ jobs:
           cmake -S . -B build/mlx-generate -DOLLAMA_MLX_BACKENDS=cuda_v13
           cmake --build build/mlx-generate --target ollama-mlx-generate-wrappers
           git diff --exit-code -- \
-            x/imagegen/mlx/mlx.h \
-            x/imagegen/mlx/mlx.c \
-            x/mlxrunner/mlx/generated.h \
-            x/mlxrunner/mlx/generated.c \
-            x/mlxrunner/mlx/include/mlx/c
+            mlx/generated.h \
+            mlx/generated.c \
+            mlx/include/mlx/c
       - name: Run go generate
         run: go generate ./...
 
@@ -484,7 +483,7 @@ jobs:
         with:
           path: .cache/mlx-darwin-release
           # Key on every payload input so a source-built payload survives pushes.
-          key: mlx-darwin-${{ hashFiles('MLX_VERSION', 'MLX_C_VERSION', 'cmake/local.cmake', 'cmake/apply-git-patches.cmake', 'cmake/mlx/CMakeLists.txt', 'cmake/mlx/CMakePresets.json', 'x/mlxrunner/mlx/CMakeLists.txt', 'mlx/compat/**', 'x/mlxrunner/xgrammar/native/**') }}
+          key: mlx-darwin-${{ hashFiles('MLX_VERSION', 'MLX_C_VERSION', 'cmake/local.cmake', 'cmake/apply-git-patches.cmake', 'cmake/mlx/CMakeLists.txt', 'cmake/mlx/CMakePresets.json', 'mlx/CMakeLists.txt', 'mlx/compat/**', 'mlxrunner/xgrammar/native/**') }}
       - name: Prepare MLX Darwin release payload
         if: ${{ startsWith(matrix.os, 'macos') }}
         run: .github/scripts/prepare_mlx_darwin.sh