feat: add HeaderMotion.SubHeader for tab-local sticky subheaders

[centricity-kashifshaikh]

Summary

• adds HeaderMotion.SubHeader compound component
• supports scrollId, dynamic height measurement, optional height hint, and topInset
• auto-applies scrollable offset integration (header + subheader + topInset)
• adds dedicated examples: subheader-flatlist and subheader-pager

Why

• enables tab-specific sticky sub-headers requested in Sticky element inside tab (not header) support #21

Validation

• yarn typecheck
• manual validation in both new example screens

Closes #21

[pawicao]

Thank you for the substantial work here, examples included, which I appreciate.

As I described in the issue itself, after review, I've decided not to take this into the library. The main reason is conceptual: the low-level API makes no assumptions about how the header animates. SubHeader has to assume one: it computes its top from originalHeaderHeight - progress × threshold, which only holds for the standard translate collapse. A header with a custom motion would leave the sub-header in the wrong place, and that's a problematic contract.

The use case itself is real, and the v2 high-level API (#27) is where I'd like to solve it properly, likely as a per-tab pinned concept. Until then, the userland pattern from #21 (absolute position + useMotionProgress + extra content padding) covers it.

Closing, but genuinely thank you for engaging with the library.