fix(cmake): export external flatcc runtime dependencies

[bmehta001]

Problem

With NANOARROW_IPC=ON and an external flatcc runtime, the exported static IPC target contains $<LINK_ONLY:flatccrt>, but the package does not define that target for its consumers. Standalone consumers therefore depend on incidental caller-defined targets or linker search paths. This affects build-tree exports as well as installed packages.

The vcpkg port currently compensates on Windows by replacing this dependency with a Release-only library path; Unix consumers are left with the bare library name. A source-level fix lets package maintainers remove that workaround when updating nanoarrow.

Related to #922. This addresses exported dependency metadata, separately from #949's build-time static/shared runtime discovery.

Changes

• Give external runtime targets the actual name nanoarrow::flatccrt, rather than an alias of an unqualified imported target. CMake resolves imported aliases back to the underlying name when exporting them.
• Preserve the existing vendored runtime export by adding its corresponding build-tree alias, and consistently use the namespaced target in IPC libraries and tests.
• Resolve external Release/Debug runtime libraries during package discovery, using the package prefix, configured external locations, and consumer prefix paths. Report a normal package-not-found result when the runtime is unavailable.
• Add a dependency-free regression driver and Windows/Linux/macOS CI coverage. It builds a real flatcc runtime from the existing vendored sources, then compiles and runs actual package consumers.
• Document installed-package consumption and explicit runtime overrides.

Validation

Windows (Visual Studio 2026 / MSVC 19.51) and Linux (WSL / GCC 13.3):

• 24 IPC consumer executions passed per platform: external root, separate include/library directories, and vendored runtime; Debug and Release; build-tree and relocated installed packages; caller-defined flatccrt declared both before and after package discovery.
• All three Debug build modes also passed with nanoarrow's default extra warnings enabled.
• Additional Debug/Release consumers passed with flatcc in a separate relocated prefix. Assertions checked the selected configuration-specific runtime path.
• Missing-runtime package discovery returned the expected failure, including with find_package(... QUIET).
• Core-only Debug/Release consumers passed without creating or requiring a flatcc target.
• The same regression driver on unmodified upstream failed with flatccrt-NOTFOUND, demonstrating that the test detects the original export defect.

Regression entry point, from the repository root:

python ci/scripts/test_cmake_package.py --build-dir build/package-tests

On Windows the local run explicitly selected CMAKE_GENERATOR=Visual Studio 18 2026.

CMake formatting (0.6.13), Black (22.3.0), Flake8 (7.1.1), isort (5.12.0), codespell, workflow YAML parsing, and git diff --check passed. No runtime tests in these selections were skipped or excluded. Native macOS was not available locally; the added CI job covers it.

AI assistance: GitHub Copilot assisted with implementation and regression coverage.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.24%. Comparing base (ea215b0) to head (b84a944).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #963   +/-   ##
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  Coverage   78.24%   78.24%           
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  Files         106      106           
  Lines       16867    16867           
  Branches     1993     1993           
=======================================
  Hits        13198    13198           
  Misses       2437     2437           
  Partials     1232     1232           

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