ACE export translation-unit split plan

src/abi/ace_exports.cpp contains the public C ABI and tightly coupled process state. Splitting it is a refactor, not a behavior change. The work must preserve exported names, ordinals, calling conventions, handle lifetime, last-error behavior, and recursive-lock re-entry.

Boundary introduced in this release

Shared process state now lives in abi/runtime.{h,cpp} under openads::abi::detail. There is still exactly one recursive mutex, handle registry, connection map, and remote-find registry. This small extraction creates a stable internal seam without moving public entry points.

Planned modules

1. backend_dispatch and remote_state
2. low-coupling export groups: management, filesystem, compatibility, and stubs
3. connection and teardown paths
4. table, field, index, and transaction exports
5. dictionary and access-control exports
6. SQL statement state, catalogs, procedures, scripts, triggers, and execution

SQL moves last because execution deliberately re-enters public SQL entry points while the recursive ABI lock is held.

Rules for each move

• Keep C signatures and ENTRYPOINT declarations byte-for-byte compatible.
• Do not combine relocation with behavior changes or cleanup.
• Keep one definition of every registry, mutex, and thread-local.
• Preserve .def manifests and x86 stdcall aliases.
• Compare produced binary exports against the release baseline in CI.
• Run Windows MSVC and MinGW plus Linux and macOS checks for every move.
• Remove /bigobj and -Wa,-mbig-obj only after the remaining translation units no longer need them.

Prefer reviewable moves of roughly 500-1,500 lines. Access-control semantics and known parity gaps are separate feature work and must not be hidden inside this refactor.