feat: add text as a renderable item

BONUS: add font support too

The issue only describes adding text as a renderable item, with font support as a bonus; it names no files, tests, or entry points. Start by tracing how existing renderable items are defined and drawn, then identify the project's current rendering and asset-loading boundaries. Done should include a defined text-rendering path and font support only if its scope is decided.

索引モデルが issue の本文から書いたものです。