feat(backend): portable process identity for Windows

[loss-and-quick]

Summary

First step of bringing Windows back as a desktop target (it was dropped because the
backend was POSIX-only). This PR makes the neutral backend build on Windows by
porting the one OS-specific piece — process identity in crates/kasumi-backend/src/proc.rs.

proc.rs was POSIX-only: it matched a pid by /proc/<pid>/exe dev/ino and terminated
via libc::kill. The OS-specific half now lives in a cfg-gated imp module:

• POSIX keeps the /proc inode match + SIGTERM→SIGKILL graceful teardown.
• Windows matches a pid by its full image path (QueryFullProcessImageNameW) and
  ends it with TerminateProcess. There is no SIGTERM, but the wintun driver reclaims a
  core's tun adapter on process exit, so the hard kill stays clean for a graceful stop.

run/silent/spawn_logged/read_pidfile were already portable and stay shared.
libc moves under cfg(unix); windows-sys (Win32 Threading + Foundation) comes in
under cfg(windows).

The rust CI job runs on Linux and never compiles the #[cfg(windows)] path, so a new
windows-check job clippy-checks the backend on a Windows runner — otherwise the Win32
code would ship unverified. The full src-tauri desktop bundle (tun + routing Platform)
stays Linux-only and is deferred to the next PR.

Affected layer

• frontend/ — React Web UI
• module/ — Magisk/KernelSU/APatch payload (shell + packaged kasumi-proxy backend)
• scripts/ — build / release helpers
• CI / .github/
• Docs only

Verification

Rust (nix devshell, Linux):

• cargo fmt --all --check — clean
• cargo clippy --workspace --all-targets -- -D warnings — clean
• cargo test --workspace — green (proc:: tests pass; shell/proc-only tests gated to unix)
• codegen drift: cargo run -p kasumi-desktop --bin codegen → git diff on frontend/src/generated empty

Windows backend compile is exercised by the new windows-check CI job (clippy on
windows-latest). Local cross-check isn't possible here — the nix toolchain ships only
the host std, no x86_64-pc-windows-msvc std.

Web UI (frontend/): n/a — no frontend change.

Module shell (module/): n/a — no shell change.

Checklist

• Title is a scoped Conventional Commit; commits are logically split
• No build artifacts committed
• If user-visible strings changed: n/a — none changed
• Renames touching the project id were grepped in all case forms — n/a, no renames

Notes for reviewers

• Windows pid_matches_bin compares the full image path (no inode on Windows):
  both sides are canonicalized, falling back to a case-insensitive compare when the
  file is no longer openable (process already exited).
• kill's graceful flag is a no-op on Windows by design — there is no SIGTERM; the
  wintun adapter teardown happens on process exit regardless.
• Part of the Windows-support series: PR 1/3 (backend) → PR 2 (WindowsPlatform +
  wintun packaging + nightly artifact) → PR 3 (promote to release.yml + README).

[loss-and-quick]

Superseded: the work is now one Windows-support PR on feat/windows-support (renamed branch, full WindowsPlatform + packaging in one PR).