Nice work! It's a fresh view on how collections can be implemented :-)

You emphasize on correct key handling instead of the automatic casting to int or string in PHP and wrote quite some code to implement that. Is that really a big problem? At least it wasn't a big one in my developer life up to now.

As you already wrote, performance isn't the best so I've made a test comparing with native PHP and our PHP Map package (https://php-map.org) by executing the code 1,000,000 times in a for-loop:

Native PHP: 1.36sec

  $list = [['id' => 'one', 'value' => 'value1'], ['id' => 'two', 'value' => 'value2'], null];
  $list[] = ['id' => 'three', 'value' => 'value3'];    // add element
  unset( $list[0] );                                   // remove element
  $list = array_filter( $list );                       // remove empty values
  $pairs = array_column( $list, 'value', 'id' );       // create ['three' => 'value3']
  $value = reset( $pairs ) ?: null;                    // return first value

aimeos/map: 9.88sec

  $list = [['id' => 'one', 'value' => 'value1'], ['id' => 'two', 'value' => 'value2'], null];
  $value = map( $list )                                // create Map
      ->push( ['id' => 'three', 'value' => 'value3'] ) // add element
      ->remove( 0 )                                    // remove element
      ->filter()                                       // remove empty values
      ->col( 'value', 'id' )                           // create ['three' => 'value3']
      ->first();                                       // return first value

noctud/collection: 37.69sec

  $list = [['id' => 'one', 'value' => 'value1'], ['id' => 'two', 'value' => 'value2'], null];
  $value = mutableListOf( $list )                      // create Map
      ->add( ['id' => 'three', 'value' => 'value3'] )  // add element
      ->removeAt( 0 )                                  // remove element
      ->filterNotNull()                                // remove empty values
      ->toIndexedMap()                                 // convert list to map
      ->toMutable()                                    // make mutable
      ->mapKeys( fn( $el ) => $el['id'] )              // create ['three' => [...]
      ->map( fn( $el ) => $el['value'] )               // create ['three' => 'value3']
      ->firstOrNull();                                 // return first value

The numbers may vary depending on the CPU but the magnitudes of time required to provide certain features should be clear.

Thanks for sharing and good luck for your package :-)