fix: use discrete palette value steps

[Woodii1998]

Stack

This PR is intentionally based on #85 so its timeout-estimator tests use the same 90-second PC ACK policy. After #85 merges, this PR can be retargeted to main without carrying unrelated changes.

Summary

• replace the custom-palette value-axis analog hold with one DpadDown press per value step
• remove the coarse/fine value split and its unused constants
• make the desktop raw duration estimator count the same discrete actions
• keep the PC <slot> #RRGGBB protocol, preview quantization, and saturation movement unchanged

Root cause

The preview assumed a linear 112-step value scale, but firmware represented all except the last 12 steps as one full-stick hold. The game accelerates continuous stick movement, so dark colors overshot their target and became darker than the preview. Discrete presses use the same one-step behavior already used for the final dark-value corrections.

Trade-off

Dark custom colors take longer. At the default 100/100ms input timing, pure black's value phase grows from 4.5s to 22.4s. The desktop estimator is updated by the same amount, and #85's 90-second ACK floor covers the resulting command duration.

Validation

• TDD RED: firmware analog-path assertion and old timing values failed before implementation
• npm run ci:local:quick — desktop 100/100, flasher 8/8, type checks and web build passed
• PlatformIO builds — Switch, Switch 2, and Switch Lite all succeeded
• task and final reviews found no blocking findings

Physical console verification of dark shades remains recommended because CI cannot read the in-game palette cursor.

Fixes #81