The completion popup (e.g. typing `/model`) grew from 8 rows at
compIdx=0 up to 16 rows at compIdx≥8 — the slice end was `compIdx + 8`
so every arrow-down added another rendered row until the window filled.
Reported during TUI v2 retest: "as i scroll and more options appear,
for some reason more options appear and it expands the height".
Fixed viewport (`COMPLETION_WINDOW = 16`) centered on compIdx, clamped
so it never slides past the array bounds. Renders exactly
`min(WINDOW, completions.length)` rows every frame.
34f24daa8d