opam: gate core/core_bench/core_unix to non-Windows (bench-only deps)

[tiensonqin]

Summary

Gate core, core_bench, core_unix opam deps behind {os != "win32"}.

These only back bench/bench.exe, which opam's dune build -p mldoc @install never compiles (no public_name, not in @install/@runtest targets). Jane Street core* packages are unavailable on win32 (core_unix, core_extended, base_bigstring, ocaml_intrinsics all carry os != "win32" availability), so the unconditional dep made mldoc unsolvable when installed as a git-pinned dep on Windows — this blocks the logseq db-worker native daemon's Windows build.

Resolution on macOS/Linux is unchanged (os != "win32" is true → same dep set).

Link to Devin session: https://app.devin.ai/sessions/62087b2a7a364e878f71f03c7ca1302d
Open in Devin Desktop: https://app.devin.ai/desktop/session/62087b2a7a364e878f71f03c7ca1302d?variant=devin
Requested by: @tiensonqin

[devin-ai-integration]

I'll fix CI failures and address comments from users with write access. I'll skip comments containing "(aside)".

• Disable automatic comment, CI, and merge conflict monitoring

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