fix(ios): avoid installTap sample-rate assertion crash when starting stream recording

[striver2006]

Addresses #622 (full root-cause analysis there).

RecorderStreamDelegate.start() can abort the whole app with an uncatchable NSException (required condition is false: format.sampleRate == hwFormat.sampleRate) when installTap runs while an asynchronous hardware sample-rate switch (session reconfiguration, Bluetooth HFP/A2DP negotiation, route change) is still in progress and inputFormat(forBus:) returned a stale format.

Changes (pure Swift, no API change)

1. Configure the session before creating the engine. Upstream order is engine → initAVAudioSession; since session configuration (category/activation/preferred sample rate) is what triggers the async hardware switch, creating the engine afterwards removes the structural half of the race.
2. Wait for the input format to settle before installTap. Poll until the format is non-zero and (unless voice processing is enabled, where the input node format may legitimately differ) matches AVAudioSession.sampleRate. The engine is re-created per retry because the input node caches the first format it observed. Bounded at 10×25 ms on the plugin's background serial queue — in normal operation it takes 0 rounds, so the happy path is unchanged. If no valid format shows up, a RecorderError is thrown instead of tripping the AVFAudio assertion.

Testing

• Device-matrix regression of our voice-interaction flow (repeated stream starts with Bluetooth headset connect/disconnect and wired-headphone plug/unplug loops): the assertion crash no longer reproduces; the format-settle loop measures 0 wait rounds in normal operation.
• This change (as part of a vendored fork) is rolling out in our production app for children's voice exercises; the crash signature came from real production crash reports against record_ios 2.1.1.

Note: a third, defense-in-depth layer (ObjC @try/@catch around installTap/prepare/start so any residual race becomes a catchable PlatformException instead of SIGABRT) is deliberately not included — it needs an ObjC file and a single SPM target can't mix languages. See #622 for discussion.

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