fix: reject same-direction duplicate sessions in addPeer (followup to v0.3.79 PR #4)

[sym-bot]

Summary

Followup to #4 (v0.3.79). The dual-dial dedup introduced there applied the nodeId tie-break unconditionally whenever a prior bonjour transport existed. The tie-break assumes the prior is in the opposite direction of the new session — true for genuine simultaneous-dial collisions, but NOT for same-direction duplicates.

When two inbound sessions arrive in rapid succession (TCP retry, multipath race, repeated newConnectionHandler firing for the same advertised service), the tie-break was applied as if the prior were an outbound. For the higher-nodeId peer this returns preferNew=true, replacing the established healthy inbound with the duplicate. The replaced session was force-disconnected; its remote wire pair saw EOF; removeTransport fired on the remote side; peer-left storm continued.

Field evidence (iPhone↔Mac Catalyst on v0.3.79)

iPhone (higher nodeId) log shows the bug — two consecutive "replacing prior with new inbound" lines:

[SYM] session: handshake complete with Michael (019dd87c)
[SYM] peer: simultaneous-dial dedup — replacing prior with new inbound to Michael
[SYM] session: handshake complete with Michael (019dd87c)
[SYM] peer: simultaneous-dial dedup — replacing prior with new inbound to Michael    ← second one

The second "replacing prior" is tearing down the survivor of the first — exactly the symptom described above.

Mac (lower nodeId) consequence — peer flap continued:

[SYM] session: handshake complete with Hongwei (019dd87d)
[SYM] peer: connected: Hongwei (outbound, bonjour)
[SYM] wake: from Hongwei: apns
SYM peer joined: Hongwei (total: 1)
SYM metric: peer-joined
[SYM] session: disconnected: Connection closed         ← survivor torn down
[SYM] peer: disconnected: Hongwei
SYM peer-left pending for Hongwei — waiting 6s for reconnect

Fix

In addPeer, only apply the dual-dial tie-break when prior and new differ in direction. Same-direction prior → keep established session, reject duplicate. The duplicate's delegate is detached before disconnect() so its teardown can't ripple back through removeTransport.

let isDualDial = prev.isOutbound != session.isOutbound
let preferNew: Bool
if isDualDial {
    preferNew = SymNode.preferNewSessionInDualDial(...)
} else {
    preferNew = false  // same-direction → keep established prior
}

Tests

New test testSameDirectionDuplicatesAreRejected — truth-table coverage of both same-direction cases (both inbound / both outbound) for both nodeId orderings, plus a regression guard that the dual-dial tie-break still applies correctly through the combined decision.

71/71 tests pass.

Test plan

• All existing unit tests pass (swift test)
• New same-direction test passes
• Verified on iPhone↔Mac Catalyst pair — single "peer connected" per cycle, no consecutive "replacing prior with new inbound" lines, no peer-left storm

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