examples: add whisper-mic example with docs and unix-only gating

[nickhuang99]

Summary

This PR integrates a new whisper-mic example for interactive microphone transcription and aligns it with existing project conventions.

What Changed

• Added examples/mic/whisper-mic.cpp as a buildable example target.
• Refactored argument handling to match the style used in other examples (help/options parsing).
• Improved runtime robustness:
  • model/context/device initialization checks
  • empty-audio guard before inference
  • error handling on recognition failure
• Improved stop behavior:
  • progress display now stops immediately when recording is stopped
  • short tail capture is kept to avoid clipping the last syllable
• Aligned mic CMake integration with upstream example style.
• Added mic documentation and listed the example in the root examples table.
• Limited mic example build inclusion to Unix-like platforms (UNIX AND NOT WIN32) to keep Windows builds clean for now.

Validation

• CPU build:
  • cmake --build build_mic --target whisper-mic
• GPU build:
  • cmake --build build_gpu_mic --target whisper-mic
• Runtime verification on Linux:
  • confirmed model load and successful microphone transcription
  • confirmed CUDA backend initialization in GPU build

Notes

• Windows support for this specific mic example is intentionally deferred; the target is currently gated out on Win32.