fix(mobile): close duplicated TUN fd after engine.Stop to prevent fd leak

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Problem

StartTun and StartTunBridge call dupFd(int(fd)) before handing the
duplicated fd to engine.Insert(key). The dup is necessary to avoid a
SIGSEGV race with Android's ParcelFileDescriptor.close() (commit 41c3eef).

However, the dup'd fd is never closed by the Go side. engine.Stop() closes
the fd it registered — but only on the happy path where _defaultDevice is
non-nil. If engine.Stop() panics (the recover() at mobile.go:282/328
swallows it) or engine.Start() never completed, the fd leaks. On Android,
where users hot-connect profiles many times per session, this leaks one fd per
cycle until the kernel refuses (EMFILE — "VPN interface could not be
established").

Fix

• Add tunOwnedFd int32 = -1 (guarded by engineMu) to track the most
  recently dup'd fd inserted into the engine.
• StartTun / StartTunBridge: defensively close any pre-existing tracked fd
  before re-inserting, then record the new safeFd.
• StopTun / StopTunBridge: explicit syscall.Close(tunOwnedFd) + reset to
  -1 after the recover()-wrapped engine.Stop().
• The load-bearing recover() (commit 41c3eef) is preserved in both Stop
  functions.
• StopTunBridge's engineMu.Unlock() → tun.StopFakeDNSProxy() ordering is
  preserved.

On the happy path (tun2socks closes the fd via _defaultDevice.Close()), the
explicit close is a harmless double-close (EBADF is ignored). The benefit is
on the unhappy paths where tun2socks does NOT close the fd.

Files changed

• mobile/mobile.go — +25/-1

Verification

• go vet ./mobile/... passes (on Linux target; Windows reports false-positive
  on syscall.Dup which is unix-only)
• gofmt -l mobile/mobile.go — clean for this patch (residual pre-existing
  issues in surrounding code are out of scope)
• gomobile bind + gradlew assembleDebug verified via CI

Test plan

No new unit tests. Runtime verification: connect/disconnect a VPN profile ~30
times on a real device and confirm no EMFILE / "VPN interface could not be
established" error.

Closes plan 004.