Fix cmake binary output dir + enable build CI on PRs

[Copilot]

When acestep.cpp is built via cmake, ggml's CMakeLists.txt silently redirects all executables to ${CMAKE_BINARY_DIR}/bin when used as a subdirectory — so ace-qwen3 and dit-vae land in build/bin/ instead of build/. Every binary search in this repo only looked in build/, so builds appeared to produce nothing.

Additionally, the build-test CI job was gated on workflow_dispatch only, meaning the fix was never automatically verified on PRs.

Changes

install.py + nodes.py — fix binary output directory

• Pass -DCMAKE_RUNTIME_OUTPUT_DIRECTORY={build_dir} to cmake so new builds always emit binaries into build/ directly
• Add _binary_exists (install.py) and _binary_in_build (nodes.py) helpers that check both build/ and build/bin/ — preserving compatibility with existing installations built without the flag
• get_binary_path() and AcestepCPPBuilder.build() both use _binary_in_build for consistent fallback search

# ggml CMakeLists.txt when used as subdirectory:
if (NOT CMAKE_RUNTIME_OUTPUT_DIRECTORY)
    set(CMAKE_RUNTIME_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/bin)  # ← the culprit
endif()

.github/workflows/ci.yml — actually test the build

• Removed if: github.event_name == 'workflow_dispatch' gate from build-test; it now runs on every PR and push to main
• Updated "Verify binaries exist" shell check to mirror _binary_in_build: checks build/$bin then falls back to build/bin/$bin

Tests

• tests/test_install.py — new, covers _binary_exists for both search paths
• tests/test_nodes.py — TestBinaryInBuild and TestGetBinaryPath covering the helper and config/PATH/not-found lookup chain

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