Raise the default macOS deployment target to 12.0

[satoshi-assistant]

Why

Xcode 27 and newer reject any deployment target below 12.0 when configuring a project. Because the wrapper default was 11.0, every VST3/AU configure step fails on an up-to-date toolchain with:

error: The macOS deployment target 'MACOSX_DEPLOYMENT_TARGET' is set to 11.0, but the range of supported deployment target versions is 12.0 to 27.0.x.

What

• Raise the default to 12.0 in the Rust build operations and in clap_wrapper_builder/CMakeLists.txt.
• Centralize the value in wrac_build_ops::util::macos_deployment_target() so the Rust build and the wrapper configure cannot disagree about the minimum macOS version.
• Drop the MACOSX_DEPLOYMENT_TARGET override from the CI workflow. It duplicated the default and silently pinned an older minimum whenever the code default changed.

MACOSX_DEPLOYMENT_TARGET still overrides the default, so callers that need a different floor keep working.

Verification

• cargo clippy -p wrac_build_ops --all-targets -- -D warnings
• cargo test -p wrac_build_ops
• Built a VST3 and an AU wrapper against this branch on Xcode 27; both succeeded and the resulting bundles report minos 12.0.