feat(asr): add pluggable ONNX streaming backend and model registry

Add a second backend (sherpa-onnx) alongside llama.cpp, so native
streaming models can be selected by ID from a JSON registry. All such
models share one adapter: only the model files and a few options differ.
Models now live under the app data directory and are fetched by
scripts/fetch-models.sh. Adding a model is a data change, not code.

Measured on the same audio: the streaming backend is ~35x cheaper in CPU
than Qwen3-ASR (0.10-0.12 vs 4.27 core-seconds per audio second), shows
the first caption at 0.48-1.44s, and matches its accuracy for zh/en.

新增 sherpa-onnx 流式后端，与 llama.cpp 并存；模型按注册表 id 从应用
数据目录加载。同族模型共用一个适配器，新增模型只改 data/models.json。
实测流式后端 CPU 约为 Qwen3-ASR 的 1/35，首个字幕 0.48~1.44s，中英
准确率持平。注意预编译库是旧 C++ ABI，必须走其 C API。

CMakeLists.txt

@@ -62,6 +62,26 @@ if(NOT LC_MTMD_LIB)
 endif()
 message(STATUS "mtmd: ${LC_MTMD_LIB}")
 
+# ---------------------------------------------------------------------------
+# sherpa-onnx（X-ASR / zipformer / Nemotron 等流式 ONNX 模型）
+#
+# 注意：官方预编译库是用旧 C++ ABI 编译的，所以只能用它的 C API。
+# 安装：把官方的 sherpa-onnx-vX.Y.Z-linux-x64-shared.tar.bz2 解到某个目录，
+#       用 -DSHERPA_ONNX_ROOT=<dir> 指定（默认见下）。
+# ---------------------------------------------------------------------------
+set(SHERPA_ONNX_ROOT "$ENV{HOME}/.local/opt/sherpa-onnx" CACHE PATH "sherpa-onnx 安装目录")
+set(LC_HAS_SHERPA OFF)
+if(EXISTS "${SHERPA_ONNX_ROOT}/include/sherpa-onnx/c-api/c-api.h")
+    find_library(LC_SHERPA_C_API NAMES sherpa-onnx-c-api
+                 HINTS "${SHERPA_ONNX_ROOT}/lib" REQUIRED)
+    find_library(LC_SHERPA_ORT NAMES onnxruntime
+                 HINTS "${SHERPA_ONNX_ROOT}/lib" REQUIRED)
+    set(LC_HAS_SHERPA ON)
+    message(STATUS "sherpa-onnx: ${SHERPA_ONNX_ROOT}")
+else()
+    message(WARNING "未找到 sherpa-onnx（${SHERPA_ONNX_ROOT}），流式 ONNX 后端将不可用")
+endif()
+
 # ---------------------------------------------------------------------------
 # core：全部业务逻辑，刻意不依赖 Qt（方便测试与复用）
 # ---------------------------------------------------------------------------
@@ -72,6 +92,12 @@ add_library(livecaption_core STATIC
     src/core/streamingasr.cpp
     src/core/livepipeline.cpp
 )
+if(LC_HAS_SHERPA)
+    target_sources(livecaption_core PRIVATE src/core/sherpaasr.cpp)
+    target_compile_definitions(livecaption_core PUBLIC LC_HAVE_SHERPA=1)
+    target_include_directories(livecaption_core PRIVATE "${SHERPA_ONNX_ROOT}/include")
+    target_link_libraries(livecaption_core PUBLIC ${LC_SHERPA_C_API} ${LC_SHERPA_ORT})
+endif()
 target_include_directories(livecaption_core PUBLIC src/core)
 target_link_libraries(livecaption_core PUBLIC
     Threads::Threads
@@ -81,6 +107,20 @@ target_link_libraries(livecaption_core PUBLIC
     ${LC_MTMD_LIB}
 )
 
+# ---------------------------------------------------------------------------
+# models：模型注册表与存储（Qt；注册表编译进二进制）
+# ---------------------------------------------------------------------------
+add_library(livecaption_models STATIC
+    src/models/modelstore.cpp
+)
+target_include_directories(livecaption_models PUBLIC src/models)
+target_link_libraries(livecaption_models PUBLIC Qt6::Core)
+qt_add_resources(livecaption_models "registry"
+    PREFIX "/"
+    BASE "${CMAKE_CURRENT_SOURCE_DIR}"
+    FILES data/models.json)
+
+
 # ---------------------------------------------------------------------------
 # ui：悬浮窗与 Qt 适配层
 # ---------------------------------------------------------------------------
@@ -97,15 +137,15 @@ target_link_libraries(livecaption_ui PUBLIC livecaption_core Qt6::Widgets)
 # GUI：悬浮字幕窗
 add_executable(live-caption apps/gui/main.cpp)
 target_include_directories(live-caption PRIVATE apps/common)
-target_link_libraries(live-caption PRIVATE livecaption_ui)
+target_link_libraries(live-caption PRIVATE livecaption_ui livecaption_models)
 
 # CLI：文件 / 流式文件 / 命令行实时；不链接 Qt Widgets
 add_executable(live-caption-cli apps/cli/main.cpp)
 target_include_directories(live-caption-cli PRIVATE apps/common)
-target_link_libraries(live-caption-cli PRIVATE livecaption_core Qt6::Core)
+target_link_libraries(live-caption-cli PRIVATE livecaption_core livecaption_models)
 
 if(CMAKE_CXX_COMPILER_ID MATCHES "GNU|Clang")
-    foreach(_t livecaption_core livecaption_ui live-caption live-caption-cli)
+    foreach(_t livecaption_core livecaption_ui livecaption_models live-caption live-caption-cli)
         target_compile_options(${_t} PRIVATE -Wall -Wextra -Wno-unused-parameter)
     endforeach()
 endif()