fix: remove default() and new() where not cross-platform compatible

[roderickvd]

Revert most of #1211 where Default and new were not cross-compatible and/or non-idiomatic for private use.

[Copilot]

Pull request overview

This PR removes recently-added Default impls and new() constructors from several host/backend-specific types where they’re not meant to be relied on cross-platform, and adjusts call sites accordingly (e.g., constructing devices/iterators directly instead of via Default/new()).

Changes:

• Removed Default derives / impls and new() constructors from various backend Host/Device/iterator types (WebAudio, AudioWorklet, Null, WASAPI, CoreAudio macOS/iOS, AAudio, ALSA).
• Replaced Default-based construction with explicit construction (e.g., Devices(is_available) / Device(None)).
• ALSA default device creation now reuses the host’s existing context instead of creating a new one via Default.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/host/webaudio/mod.rs	Removes Default/new() and constructs Devices explicitly based on WebAudio availability.
src/host/wasapi/mod.rs	Removes Default derive from WASAPI Host.
src/host/null/mod.rs	Removes Default/new() from Null backend types and updates devices() accordingly.
src/host/coreaudio/macos/mod.rs	Removes Default derive from CoreAudio macOS Host.
src/host/coreaudio/ios/mod.rs	Removes Default/new() usage and constructs Devices directly (currently introduces a privacy/constructability issue).
src/host/coreaudio/ios/enumerate.rs	Removes Devices::new()/Default for iOS enumerator iterator.
src/host/audioworklet/mod.rs	Removes Default/new() and constructs Devices explicitly from availability.
src/host/alsa/mod.rs	Removes Default for ALSA Device; adds Host::default_device() using the host context.
src/host/aaudio/mod.rs	Removes Default/new() for AAudio Device and replaces call sites with Device(None).
CHANGELOG.md	Updates changelog entries to reflect removed Default/new() additions and related fixes.

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