Step 14: the brotli encoder, levels 1 and 5 #8

Design §8 step 14, from docs/design.md as the owner ruled it on 2026-09-25. The step's entry in design §8 records what each check printed when it passes; this issue tracks the work.

Step 14: the brotli encoder. Levels 1 and 5, after a decision record of its own on levels and memory.

Check: as step 9, through Google's brotli and stdx's decoder.

Waits on: step 12, and a decision record of its own on levels and memory before any code.