feat: build and run on Windows

[tamirms]

The core read/write paths used unix.Pwrite/Pread on raw fds, so the library failed to compile on Windows — breaking downstream importers there.

This swaps them for portable *os.File.WriteAt/ReadAt and routes the !linux && !darwin prealloc fallback through os.File.Truncate (no unix import). The unsorted builder's anonymous temp files use unlink-while-open on Unix and FILE_FLAG_DELETE_ON_CLOSE on Windows, so the whole library builds and runs on Windows.

Path	Unix	Windows
Query/read + sorted builder	✅	✅
Unsorted builder	✅	✅ (FILE_FLAG_DELETE_ON_CLOSE)

On Unix WriteAt is the same pwrite syscall and keeps the lock-free concurrent-write property. Build throughput unchanged within ~5% noise (10M keys, ptrhash).

Verified: go test -race ./..., gofmt/vet/golangci-lint, and GOOS=windows/freebsd cross-compile.

Not verified / known limitations (CI is ubuntu+macos only, so the Windows runtime path is unexercised):

• Windows temp-dir cleanup is best-effort. The DELETE_ON_CLOSE files are reclaimed on close (and on crash), but the temp directory is removed in cleanup(), so an abnormal exit leaks an empty streamhash-parts-* dir. A handle held by AV/indexer at close time can also leave the dir briefly in delete-pending and skip removal.
• A Windows CI runner is the recommended follow-up before relying on the Windows runtime path.

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR removes golang.org/x/sys/unix usage from core read/write paths and replaces raw-fd pread/pwrite syscalls with portable *os.File.ReadAt/WriteAt, enabling the library to compile (and the sorted builder + query/read paths to run) on Windows while keeping platform-specific preallocation behind OS build tags.

Changes:

• Replaced unix.Pwrite/Pread call sites with (*os.File).WriteAt/ReadAt and adjusted APIs to pass *os.File / io.ReaderAt instead of raw fds.
• Refactored preallocFile to accept *os.File, using ftruncate-free fallback (os.File.Truncate) on !linux && !darwin and retaining fallocate/F_PREALLOCATE on Linux/macOS.
• Updated platform documentation to reflect Windows compilation support and the unsorted builder limitation.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
platform_other.go	Removes unix dependency by using os.File.Truncate for the non-Linux/Darwin prealloc fallback (covers Windows).
platform_linux.go	Updates preallocFile to accept *os.File and uses Fallocate(int(f.Fd()), ...).
platform_darwin.go	Updates preallocFile to accept *os.File and uses FcntlFstore(f.Fd(), ...).
index_writer.go	Switches positioned writes from unix.Pwrite on cached fds to file.WriteAt.
builder_unsorted.go	Switches unsorted writer flush from unix.Pwrite to WriteAt and removes cached fd state.
builder_unsorted_finish.go	Switches partition reads from unix.Pread to io.ReaderAt.ReadAt.
CLAUDE.md	Updates platform support notes to reflect Windows compilation and unsorted builder limitation.

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[Copilot]

Pull request overview

Copilot reviewed 9 out of 9 changed files in this pull request and generated 2 comments.

[Copilot]

Pull request overview

Copilot reviewed 9 out of 9 changed files in this pull request and generated 2 comments.

[chowbao]

nit: It doesn't look like the ci/cd doesn't actually check windows. I only see macos/ubuntu

[tamirms]

nit: It doesn't look like the ci/cd doesn't actually check windows. I only see macos/ubuntu

yes, this is a follow up task which has been de-prioritized but can be done later