Add random and centered usage sort orders to Tagadelic

Problem/Motivation

Descending usage order places the largest terms at the beginning. Site builders also need mixed sizes and a layout with the largest terms near the middle.

Proposed resolution

• Provide Highest usage first, Random, and Highest usage in the middle options for Tagadelic.
• Apply the term limit to the most-used terms before reordering them.
• For centered usage, place progressively smaller terms on alternating sides of the peak. Center refers to reading order, not the geometric center of wrapped lines.
• Keep random order stable while the rendered block is cached, and preserve assigned palette colors through reordering.

Remaining tasks

Split the local implementation into a focused patch or merge request, attach it here, review, and merge.

Validation

Requires the Tagadelic display issue. Verify centered ordering for odd and even term counts, random membership, empty and single-term lists, and that limited results still contain the most-used terms.