pivot: replace WinDivert engine with embedded sing-box + wintun

After 5+ hours of WinDivert NETWORK-layer NAT-rewrite debugging
(streamdump pattern, SOCKET-layer SYN preemption, lazy PID resolution,
UDP ASSOCIATE relay + manual reinject), Discord voice still wouldn't
connect. The fundamental issue is that WinDivert reinjected UDP
packets don't always reach connect()-bound application sockets — the
demux happens at a layer above the reinject point.

dvp/force-proxy avoids this entirely via DLL injection (above the
kernel demux). We avoid it the other way: embed sing-box, let it run
TUN inbound + per-process routing rule + SOCKS5 outbound. TUN packets
are read by sing-box from kernel as a normal flow; the application
socket sees a normal flow back. No reinject hairpin, no SYN race, no
spoofing concerns.

What this commit does:
  - Drops internal/divert, internal/engine, internal/redirect,
    internal/socks5, internal/procscan, plus cmd/drover/{proxy,
    debugflow}_*.go subcommands (all WinDivert-only).
  - Adds internal/sboxrun — embed sing-box.exe (1.12.25) + wintun.dll
    (0.14.1) via //go:embed, install to %PROGRAMDATA%\Drover\sboxrun\
    with SHA256 verify, generate JSON config from form, spawn as
    subprocess, manage lifecycle.
  - Wires sboxrun into internal/gui/app.go: StartEngine/StopEngine
    now call sboxrun.Engine instead of windivert engine.
  - Fixes Wails binding: StartEngine(cfg) now passes the form config
    (was zero-arg, hit ProxyHost-required validation silently).

Manual test: Discord chat + voice work end-to-end through mihomo
upstream. Yandex Music / svchost / etc continue direct via
process_name routing rule.

Binary grew from 12 MB → 49 MB (37 MB sing-box embedded), but ships
fully self-contained. AV-friendly: wintun is Microsoft-signed, no
DLL injection.

WinDivert work preserved on experimental/windivert branch in case we
ever want to come back to that path.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/sboxrun/runner_other.go

@@ -0,0 +1,48 @@
+//go:build !windows
+
+package sboxrun
+
+import (
+	"context"
+	"errors"
+)
+
+// Status — duplicate of the Windows-side enum so call sites compile.
+type Status string
+
+const (
+	StatusIdle     Status = "idle"
+	StatusStarting Status = "starting"
+	StatusActive   Status = "active"
+	StatusFailed   Status = "failed"
+)
+
+// Engine stub for non-Windows builds.
+type Engine struct{}
+
+// New returns an error on non-Windows: sing-box + wintun + WFP-based
+// per-process routing only make sense on Windows.
+func New(_ Config) (*Engine, error) {
+	return nil, errors.New("sboxrun is Windows-only")
+}
+
+func (e *Engine) Start(_ context.Context) error { return errors.New("sboxrun is Windows-only") }
+func (e *Engine) Stop() error                   { return nil }
+func (e *Engine) Status() Status                { return StatusIdle }
+func (e *Engine) LastError() error              { return nil }
+func (e *Engine) LogPath() string               { return "" }
+func (e *Engine) ConfigPath() string            { return "" }
+
+// AssetPaths stub.
+type AssetPaths struct {
+	SingBoxExe string
+	WintunDLL  string
+	WorkDir    string
+	ConfigPath string
+	LogPath    string
+}
+
+// InstallAssets stub.
+func InstallAssets() (*AssetPaths, error) {
+	return nil, errors.New("sboxrun is Windows-only")
+}