fix(sqlite-objstore): provide S_ISDIR/S_ISREG macros for MSVC

[gabewillen]

Round 5 of the Windows port for the npm release of @augmem/cortext v1.3.3
(follow-up to PRs #22-#25). Compile and the cortext.lib name collision are
fixed; both Windows matrix jobs now fail at link with:

unresolved external symbol S_ISREG
unresolved external symbol S_ISDIR

MSVC does not define the POSIX S_ISDIR/S_ISREG macros, so C treated the
identifiers as implicit functions. This adds the standard
((m) & _S_IFMT) == _S_IFDIR/REG definitions to win_posix.h (guarded) and
includes that header from backend_fs_common.c on Windows. No other S_IS*
macros are used in the embedded objstore sources.

[Copilot]

Pull request overview

This PR unblocks the Windows build/link step for the embedded sqlite-objstore backend by providing missing POSIX-style S_ISDIR / S_ISREG macros for MSVC, which otherwise treats them as implicitly-declared functions and fails at link.

Changes:

• Add guarded MSVC-compatible definitions for S_ISDIR and S_ISREG in win_posix.h using _S_IFMT / _S_IFDIR / _S_IFREG.
• Include win_posix.h from backend_fs_common.c on _WIN32 so the directory/file checks in that translation unit compile correctly on MSVC.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
third_party/sqlite-objstore/src/win_posix.h	Adds guarded S_ISDIR / S_ISREG macro definitions for Windows/MSVC builds.
third_party/sqlite-objstore/src/backend_fs_common.c	Ensures Windows builds include win_posix.h so S_ISDIR use in this TU doesn’t become an unresolved external at link time.

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