fix: audio-mix and stop/start races in both publishers

[joeblau]

Two pre-existing robustness gaps shared by RTMPPublisher and SessionPublisher, surfaced by the issue #20 resilience review (they were out of scope there — pre-existing in both, not #20 regressions). Fixed in both via the shared layer; RTMP's normal go-live path is unchanged (both guards fire only on a concurrent stop).

1. App-audio mix silent if the mic is dead from go-live

HaishinKit renders the multitrack mix only when the MAIN track (mic, track 0) appends. The mic-stall failover promoted app audio (track 1) to main only after the mic had appended at least once (lastMicAppendAt > 0) — so a mic dead from the start never promoted, and the whole mix (app audio included) stayed silent forever.

MicStallEvaluator.shouldPromoteApp now takes startedAt and measures mic silence from go-live when the mic has never appeared (micReference = lastMicAppendAt > 0 ? lastMicAppendAt : startedAt). The previously-working stall path is byte-for-byte unchanged.

2. stop() during start()'s pre-isRunning setup leaked tasks

Both publishers ran setup awaits (factory/mixer setup + startRunning) before isRunning = true and before spawning the path-supervisor / watchdog / frame-repeat tasks. A stop() interleaving there early-returned via guard isRunning, then start() resumed and spawned tasks it could never cancel — a 2s watchdog timer waking forever + the mixer left running.

Added guard !userInitiatedStop { await mixer.stopRunning(); return } after mixer.startRunning() in both publishers, plus a second re-check in RTMP after its two await connection.status/await stream.status calls (a narrower window Session doesn't have).

Verification

• StreamCore: 99 tests pass (+3 new mic-stall cases: dead-from-go-live promotes, within-grace doesn't, no-app-audio doesn't)
• Full app builds against the iOS 27 SDK
• Adversarially reviewed (mic-fix correctness · start-race completeness · RTMP normal-path preservation); the review's one residual finding (a second RTMP start window) is fixed here.
• ⚠️ Audio mixing + start/stop races are device-only-verifiable; worth a smoke test (start with mic denied + app audio on; stop mid-connect).

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