Windows support: compile and run natively on x86_64-pc-windows-msvc

[gqf2008]

Goal

Make walgit build and run natively on Windows today's host toolchain (x86_64-pc-windows-msvc, Rust 1.97.1): core production crates currently fail to compile because several Unix-only APIs are used without #[cfg] gates.

Why

The server story is Linux-first, but contributors run Windows; nothing even type-checks there today, so portability can silently rot further. Containers remain the recommended deployment shape — this issue is about the code no longer being un-buildable off POSIX.

Batch checklist

• walgit-wal/src/registry.rs: hard-link-aware dir_size (MetadataExt dev/ino/nlink) and disk_usage (libc::statvfs) are ungated Unix code — add per-OS implementations behind cfg
• Unix symlink creation ungated (walgit-wal/src/sync.rs, walgit-cli/src/import_direct.rs, walgit-server/src/rebuild.rs incl. its statvfs free-space probe) — converged into a new platform seam, crates/walgit-wal/src/platform.rs
• walgit-server/src/tls.rs: POSIX permission bits on key/session files need gating (already gated; verified)
• D39 paths: cache.dir default /tmp/walgit and the --config /dev/null sentinel are POSIX-shaped
• Tests spawning sh / using PermissionsExt / libc::rusage: gate or cross-platformize (git_pipe buffered pipes ×4 suites; symmetric cfg gates; detect-and-speak skips)
• .cargo/config.toml: GIT_CONFIG_GLOBAL=/dev/null on Windows — kept single-valued with a truthful note (git reads an unopenable global config as empty); constant-exercised by every cargo-spawned git in the suite
• CI: a Windows leg must compile/run every new cfg(windows) branch (reachability rule: no ungated-but-unbuilt branches)
• Docs reflect supported platforms

Plus one find the audit could not have predicted: remove_pack now releases this process's own pre-warmed odb index mappings before deleting and rides out a brief scanner lock with a bounded retry — refresh() mmaps every idx by design and, unlike Unix, Windows refuses to delete a mapped file.

Acceptance

1. On Windows (Win10, rustc 1.97.1 msvc): fast tier green — 185 lib/bins + 168 integration + 38 server tests, 0 failures; zero new clippy findings on touched files.
2. On Ubuntu: POSIX branches byte-for-byte unchanged by construction (#[cfg(unix)] twins of every changed seam); existing CI tiers untouched.

Non-goals left open here:

• workspace-wide clippy -D warnings fails on main independent of OS (strict-gate pedantic debt landed on main red);
• just e2e / sim suites not yet ported (the shebang-shim test is unix-only by mechanism).

Branch: windows-port on @gqf2008's fork (commit 8a5aa2e). Upstream PR awaits maintainer authorization per contributor policy.

[gqf2008]

✅ [开发完成][wt-windows-port] — branch windows-port pushed to gqf2008/walgit (8a5aa2e); fast tier 391/391 green on native Windows. Marker moves out of 处理中; the issue closes at merge.

[gqf2008]

✅ [完成][wt-windows-port] PR opened: #19 (branch windows-port from @gqf2008's fork). Marker moves out of in-progress; the issue closes when the PR merges.