fix(terminal): correct attachExisting reentrancy supersession check (W3)

[JasonYeYuhe]

What & why

Follow-up to #243/#244/#245. TerminalSessionAdapter.attachExisting(to:sessionId:) subscribes to the live raw stream, awaits the tail snapshot, then paints. To stop a rapid re-attach to a different session from clobbering the in-flight one, it checked if Task.isCancelled { return } after the await — but it never stores or cancels its own Task, so Task.isCancelled was always false. A reconnect to session B while A's snapshot was in flight flushed A's stale snapshot into B's freshly-installed reattach buffer → garbled/duplicated terminal output. (Gemini deep-review 🔴.)

Fix

The adapter is @MainActor, so the snapshot await is the only suspension point and state after resume is the authoritative supersession signal. Replaced the dead check with a pure nonisolated static predicate:

nonisolated static func reattachSuperseded(state: State, targetSessionId: String) -> Bool {
    if case let .running(sid) = state { return sid != targetSessionId }
    return true
}

attachExisting bails before painting iff reattachSuperseded(...). nonisolated + pure so it's unit-tested without a WKWebView (the XCTest host aborts on WKWebView creation outside an AppKit loop — same reason render is a manual gate).

Tests

TerminalSessionAdapterTests (W3): same running session → not superseded; different running session → superseded; every non-running state → superseded. swift test full: 1825 tests, 0 failures.

Deferred (P2, per reviewers)

• window.pushChunk WKContentWorld isolation — non-trivial index.html/TerminalView restructure; render changes need the W2 on-device smoke to validate. The TerminalNavigationGuard already blocks off-bundle navigation (defense-in-depth). Tracked separately.
• Helper-not-running UX already shipped in feat(devid): gate terminal menu on helper-reachable + localControlEnabled (W1-B) #244 (W1-B).

🤖 Generated with Claude Code

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[chatgpt-codex-connector]

Preserve cancellation for same-session reattach

TerminalAttachSession.doAttach cancels the previous attachTask before every reconnect, but reconnects reuse the same sessionId. If the canceled attach's getTailSnapshot resumes after the new same-session attach has set state back to .running(sessionId:), this state-only guard treats the canceled attach as current and falls through to paintReattachSnapshot, flushing the newer reattachBuffer before its own snapshot arrives. That reintroduces interleaved/garbled terminal output on same-session reconnects; keep Task.isCancelled (or an attach generation token) in addition to the session-id check.

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