fix: 🐛 Prevent nullptr == Tap() crash on iOS during recorder stream start

[lavigarg-simform]

Description

On iOS, starting the recorder bytes stream could crash with a fatal
nullptr == Tap() exception (issue #487).

RecorderBytesStreamEngine.attach() installed a tap on input bus 0 without
clearing any existing tap first. Two paths left a stale tap on the bus:

1. A second attach() call before detach() (re-start without teardown).
2. A prior audioEngine.start() that threw — the tap was installed but never
   removed, leaking it onto the bus.

AVAudioEngine throws nullptr == Tap() when a tap is installed on a bus that
already has one, crashing the app.

This PR makes attach() reset prior state before installing:

• Stops the engine if running, removes any existing tap on bus 0, and resets
  totalFrames.
• On a failed audioEngine.start(), removes the just-installed tap so it
  doesn't leak onto the bus and crash the next attach().

No public API change.

Checklist

• The title of my PR starts with a Conventional Commit prefix (fix:, feat:, docs: etc).
• I have followed the Contributor Guide when preparing my PR.
• I have updated/added tests for ALL new/updated/fixed functionality.
• I have updated/added relevant documentation in docs and added dartdoc comments with ///.
• I have updated/added relevant examples in examples or docs.

Breaking Change?

• Yes, this PR is a breaking change.
• No, this PR is not a breaking change.

Related Issues

Closes #487