Metric: consonance, alliteration

Consonance should score the repetition of consonant sounds
Alliteration should score the repetition of vowel sounds

Score should be represented as +1 unit per instance of alliteration/consonance, as well as relate to the text body (in second, supporting metrics) by being divided by cases _not_ of alliteration/consonance.

Both of these metrics can be combined into a rhythm score (probably with beat/phoneme detection).

No files, tests, or entry points are named. Start by locating how existing metrics are implemented and how text scores are represented, then clarify the consonance and alliteration definitions and normalization against non-matching cases. Done should include both metrics with agreed scoring and a combined rhythm score only if its scope is defined.

Written by the indexing model from the issue text.