perf(landing): gate idle repaints and slow snow tick to 2fps

[TheGreatAxios]

Summary

• The idle landing timer skips ticks that would repaint nothing while the mark is suppressed and still repaints drifting snow otherwise
• The idle repaint cadence slows from 8fps to 2fps, which still reads as motion at half a row per second of snowfall

Verification

• bun test src/tui/landing.test.ts passes (32 pass, 0 fail)
• bun run typecheck passes
• 5s headless idle: 39 repaints before, 9 after; the self-driven snow test confirms motion stays perceptible

Fixes CL-9012

[linear-code]

CL-9012