fix(engine): preserve the remote off reason during teardown #1920

Problem

The device can receive session_closed with reason owner_exited after an explicit /remote off, where the end-to-end contract expects sharing_disabled. Sharing does stop, but the client loses the reason the user requested.

Evidence

• Linux CI for PR fix(tui): show remote pairing in a full-height panel #1919 at 3a6dd8d287422778ac7c1d298d2673b94d9fba67, first attempt: failed test job.
• remote_gateway::e2e_tests::a_paired_device_drives_a_shared_session_end_to_end failed at rustcode/engine/src/remote_gateway/e2e_tests.rs:700: actual "owner_exited", expected "sharing_disabled".
• PR fix(tui): show remote pairing in a full-height panel #1919 changes pairing presentation and panel rendering; it does not modify the gateway routing, owner transport, or this test.
• The full macOS workspace suite passed on the PR branch with four test threads. The failing case passed in isolation on unchanged main (84f3d99b) on 2026-10-09. This points to intermittent behavior; a concrete root cause is not confirmed.
• The test terminal's off sends OwnerMessage::Unregister { reason: SharingDisabled } and exits its task. The owner client writes that frame and returns; the gateway uses owner_exited when the connection ends without processing a close reason. An ordering issue during teardown is a hypothesis, not a verified diagnosis.

Acceptance criteria

• Explicit /remote off preserves sharing_disabled through terminal → owner connection → gateway → device while clearing the session listing.
• Abrupt owner loss continues to report owner_exited.
• A deterministic regression covers the confirmed cause; repeated share/off cycles pass under concurrent gateway activity and on Linux CI.
• Do not weaken the assertion to accept both reasons without making that an explicit protocol decision.