QVAC-25104 fix: stop publishing platform-dependent values in fabric's package config

[jpgaribotti]

🎯 What problem does this PR solve?

• qvac-fabricConfig.cmake publishes QVAC_FABRIC_ABI_VERSION and QVAC_FABRIC_OWNS_CXX_RUNTIME, which describe the platform they were configured for — ON on a Linux release link, OFF on Android, empty on darwin, iOS and win32.
• The file carrying them is not per-platform. It installs to share/qvac-fabric/cmake, every prebuild leg produces it, and the merge job downloads all nine legs with merge-multiple: true, so the published package keeps whichever leg finished last. @qvac/fabric@0.16.0 ships nine platform directories and one prebuilds/share/qvac-fabric/cmake/qvac-fabricConfig.cmake, and qvac_addon_use_fabric points qvac-fabric_DIR at exactly that copy.
• 0.16.0 happens to carry ON, so today's consumer builds are right by artifact ordering rather than by construction — and only 2 of the 9 legs produce that value. The draw is re-rolled on every release. 0.16.1 still published them.
• The collision predates this: 0.15.0's config defined no QVAC_FABRIC_* variables and the merged qvac-fabricTargets.cmake has no platform-specific content, so share/ was effectively identical across legs. 0.16.0 was the first release to put a platform-varying value on a shared path.

📝 How does it solve it?

• Removes both variables from the published config. Publishing them per triplet was the alternative, but there is nothing left to publish for: a consumer asserts the pin against the .bare for its own triplet, where __cxa_throw is either defined under a version node or is not, so QVAC-25104 fix: assert addons pin fabric's C++ runtime and narrow exports #4519 stopped reading either variable and nothing else in the repository ever did.
• QVAC_FABRIC_ABI_VERSION stays as a build variable in CMakeLists.txt, still stamped onto the version node so the name and the script cannot drift.
• Leaves a comment in the template recording why nothing platform-dependent may go back into that file.

🧪 How was it tested?

• Generated the config through configure_package_config_file with fabric's template and loaded it from a separate consumer project: find_package(qvac-fabric CONFIG REQUIRED) resolves, the version file matches, the targets include runs, and both variables come back undefined.
• Confirmed against the published tarball that the shared path holds one merged copy, and that the non-shared, per-triplet content is untouched by this change.

💥 Breaking Changes

Removes two variables from the package config's public surface. Both were introduced in 0.16.0, describe the wrong platform for 7 of the 9 published legs, and have no consumer in this repository. The changelog entry sits under [Unreleased]; the version is deliberately not stamped here.

Rebased onto main after #4519, #4566, and the 0.16.1 addon rebuilds landed.

[github-actions]

License compliance — clean

No new dependency license findings in this PR.

Warn-only (shadow) mode — this check does not block merges yet.

Updated automatically by the canonical license compliance workflow.

NOTICE presence (advisory)

Missing NOTICE (advisory, does not block):

• ./docs/website
• ./packages/fabric/test/integration
• ./packages/llm-llamacpp/benchmarks/server
• ./packages/llm-llamacpp/benchmarks/performance
• ./packages/inference-addon-cpp/mobile
• ./packages/asr-ggml/benchmarks/server
• ./packages/embed-llamacpp/benchmarks/server
• ./packages/embed-llamacpp/benchmarks/performance
• ./packages/sdk/e2e
• ./packages/vla-ggml/sim/server
• ./.github/actions/release-merge-guard

[github-actions]

Review Status

Current Status: ✅ APPROVED
Approvals so far: Team Lead: 2, Member: 1