fix(sqlite-objstore): provide win32 unistd/fcntl shim for MSVC builds

[gabewillen]

Round 2 of the Windows port for the npm release of @augmem/cortext v1.3.3
(follow-up to PR #22, the dirent shim). After the dirent fix, both Windows
matrix jobs failed with:

backend_portable.c(17,10): error C1083: Cannot open include file: 'unistd.h'
backend_file.c(31,10): error C1083: Cannot open include file: 'unistd.h'

Audit of the real UCRT headers (not MinGW) shows MSVC/UCRT in plain C mode
provides _open/_read/_write/_close/_commit/_chsize_s/get_osfhandle,
unlink/rename/stat, and the prefixed O* flags - but not the bare POSIX
names, the un-prefixed O* aliases (gated on _CRT_DECLARE_NONSTDC_NAMES,
which is 0 in C), O_CLOEXEC, fsync, ftruncate, or flock.

This adds win_posix.h/.c (guarded so MinGW also compiles): O_* aliases,
O_CLOEXEC -> _O_NOINHERIT, open/read/write/close -> UCRT underscore
primitives, fsync -> _commit, ftruncate via _chsize_s, and flock emulated
with LockFileEx/UnlockFileEx on the 1-byte range at offset 0 (LOCK_NB ->
non-blocking with EACCES on ERROR_LOCK_VIOLATION; blocking form via a 10 ms
retry loop). The three affected files swap unistd.h / sys/file.h for the
shim under a _WIN32 include guard. Note: the flock-based recovery protocol
in backend_file.c is already #if !defined(_WIN32) (Windows runs recovery
unconditionally, as in the published v1.2.4 win32 addons), so flock is
defensive here.

Verified: 10/10 object cross-compiles (win_posix, win_dirent,
backend_fs_common, backend_portable, backend_file) for both
x86_64-windows-gnu and aarch64-windows-gnu via zig, -pedantic clean, shim
exports flock/ftruncate; native macOS arm64 objstore suite 4/4 ctest,
80/80 unit tests, warnings-as-errors clean.

[Copilot]

Pull request overview

This PR continues the Windows (MSVC/clang-cl) portability work for the embedded sqlite-objstore backend by replacing missing POSIX headers (unistd.h, sys/file.h) with a Win32/UCRT-compatible shim and wiring that shim into the build so the Windows npm release matrix can compile.

Changes:

• Add a Win32 “POSIX shim” (win_posix.h/.c) providing O_* aliases, open/read/write/close, fsync, plus flock/ftruncate implementations for MSVC/UCRT environments.
• Swap <unistd.h> / <sys/file.h> includes to win_posix.h under _WIN32 in the affected objstore backends.
• Include win_posix.c in both the vendored objstore sub-build and the top-level embedded source list on Windows.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
third_party/sqlite-objstore/src/win_posix.h	Introduces the Windows/UCRT POSIX-compatibility header for objstore backends.
third_party/sqlite-objstore/src/win_posix.c	Implements flock() (Win32 locking) and ftruncate() (via _chsize_s) for Windows builds.
third_party/sqlite-objstore/src/CMakeLists.txt	Adds win_posix.c to objstore sources when building on Windows.
third_party/sqlite-objstore/src/backend_portable.c	Uses win_posix.h instead of <unistd.h> on _WIN32.
third_party/sqlite-objstore/src/backend_file.c	Uses win_posix.h instead of <unistd.h>/<sys/file.h> on _WIN32.
CMakeLists.txt	Adds win_posix.c to the embedded objstore source list for Windows builds.

Suppressed comments (1)

third_party/sqlite-objstore/src/win_posix.c:41

• The LOCK_NB path is not actually non-blocking: LockFileEx blocks unless LOCKFILE_FAIL_IMMEDIATELY is set. Also, like UnlockFileEx, it requires a non-null OVERLAPPED*. This means a caller using LOCK_NB could hang instead of returning EWOULDBLOCK/EAGAIN.

      if (LockFileEx (handle, exclusive ? LOCKFILE_EXCLUSIVE_LOCK : 0,
                      0, 1, 0, (LPOVERLAPPED) 0) != 0)
        {
          return 0;
        }

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