fix(removeUnknownsAndDefaults): keep attributes referenced by attribute selectors

[mahirhir]

#2144 made this plugin keep an attribute when its value equals the default and it's referenced by an attribute selector such as [preserveAspectRatio]. That guard only landed in the "default value" branch.

The same loop deletes attributes in two other branches: unknown attributes, and attributes that redundantly override an inherited value. Neither one looks at the stylesheet, so both still remove attributes that a selector relies on.

For example, running only this plugin on:

<svg xmlns="http://www.w3.org/2000/svg">
  <style>rect[fill] { stroke: red; }</style>
  <g fill="blue">
    <rect fill="blue" width="10" height="10"/>
  </g>
</svg>

rect's fill="blue" repeats the parent value, so the uselessOverrides branch drops it, rect[fill] stops matching, and the rect loses its stroke. A custom attribute used as a styling hook ([state]) runs into the same thing through the unknown-attribute branch.

This applies the existing includesAttrSelector check to all three branches instead of just one. Added a test fixture covering the two cases that weren't handled.