- input/coreaudio.c, input/coreaudio.h: new HAL-output AudioUnit backend
  - source=auto captures the default output device (loopback-capable)
  - source=auto_input captures the default input device (microphone)
  - source=list enumerates all devices with input/output channel counts
  - source=<name> or <index> selects a specific device
  - Reads actual device channel count so mono devices are visualised
    correctly instead of appearing as a stereo view with a silent right channel
- configure.ac: detect CoreAudio/AudioUnit frameworks; add --disable-input-coreaudio
- Makefile.am: add COREAUDIO conditional source block
- config.h: add HAS_COREAUDIO / INPUT_COREAUDIO enum value
- config.c: register method name, default priority on macOS, source default
- cava.c: include and dispatch INPUT_COREAUDIO
- example_files/config, README.md: document coreaudio method and source options

- add Objective-C CoreAudio tap backend (input/coreaudio_tap.m)
- route coreaudio source=tap/tap_mono through tap backend
- detect tap API support in configure and add COREAUDIO_TAP conditional
- include tap sources in automake build when supported
- document tap usage in README and example config
- fix configure tap checks to use POSIX-compatible test syntax

* add coreaudio permission detection and requests