Mac: openzgy fails to compile (strerror_r signature + missing omp.h)

[magnesj]

Problem

On macOS (Homebrew llvm@19, x86_64), the vendored ThirdParty/openzgy
library fails to compile with two errors:

1. open-zgy/native/src/exception.cpp:60 — the #else branch assumes the
   glibc signature for strerror_r (returns char*), but Apple's libc
   returns int:

   char *e = strerror_r(system_errno, buf, sizeof(buf)-1);
            ^   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

2. open-zgy/native/src/impl/bulk.cpp:149 — #include <omp.h> fails
   because Homebrew llvm@19 doesn't ship libomp and
   ThirdParty/openzgy/CMakeLists.txt calls find_package(OpenMP) only as
   optional, so no include path is added. The same unconditional include
   appears in 5 more files (file_sd.cpp, mtguard.cpp, workorder.cpp,
   zgycopyc.cpp, wrappermodule.cpp).

The upstream open-zgy README documents Linux and Windows as the only
supported platforms.

Proposed fix

Adopt the brute-force patch maintained by the cssr-tools/homebrew-opm
tap, vendored as ThirdParty/openzgy/patches/macos-disable-openmp.patch
and applied automatically from ThirdParty/openzgy/CMakeLists.txt when
APPLE is true. The patch:

• Replaces #include <omp.h> with three inline omp_get_*() stubs
  returning 1/0 in 6 files. Remaining #pragma omp directives become
  no-ops because clang is invoked without -fopenmp on macOS.
• Rewrites the strerror_r fallback in exception.cpp to use a plain
  \"Unknown errno N\" string.
• Comments out a BrickIterator bounds-check in edgebrick.h and the
  exception-capture blocks in MTGuard::fail() / WorkOrderTP::fail();
  these depend on OpenMP-driven parallel regions and are unreachable
  with threading disabled.

ThirdParty/openzgy/CMakeLists.txt is updated to:

• Skip find_package(OpenMP) on APPLE.
• Apply the patch idempotently via git apply with
  --reverse --check as the "already applied" probe; emit
  FATAL_ERROR with a pointer to the patch file when the source has
  drifted and the patch needs refreshing.

All edits are inside if(APPLE) guards — Linux and Windows configure
paths are byte-identical.

Trade-offs

• OpenZGY operations on macOS run single-threaded (acceptable functional
  trade-off; performance regression on large ZGY ingest).
• The disabled bounds-check and fail() blocks should be revisited if
  OpenMP is ever re-enabled on macOS.

Alternative considered

SoilRos/homebrew-OPM ships a minimal patch that only fixes the
preprocessor branch in exception.cpp and relies on Homebrew libomp
being wired into the CMake invocation. Rejected in favour of the
self-contained brute-force approach so the build works without
brew install libomp.

Sources

• https://github.com/cssr-tools/homebrew-opm (patch origin)
• https://github.com/SoilRos/homebrew-OPM (alternative approach)

Verification

• git apply --check succeeds against the current vendored open-zgy tree
  (commit ea9a1b7d8 Update to latest open-zgy).
• After applying the patch, exception.cpp, bulk.cpp, mtguard.cpp
  and workorder.cpp compile cleanly with clang++ -std=c++20 -arch x86_64 from Homebrew llvm@19 — no strerror_r or omp.h errors.
• if(APPLE) guards leave Linux/Windows configure paths unchanged.