fix: remove tablet pressure stroke startup blot

[devmobasa]

Fixes #233

Summary

• Reseed in-progress tablet strokes with the first real pressure sample.
• Avoid treating the pre-pressure base pen width as the stroke’s pressure peak.
• Add regression coverage for pressure strokes that start before pressure data arrives.

Why

Tablet frames can deliver tip-down/motion before the first non-zero pressure sample. If the configured pen thickness is much larger than the tablet pressure range, the first stroke sample renders as a visible blot at the contact point.