fix(vidstack): read slider intersection signal

[mihar-22]

Summary

• Read the slider intersection signal value in #watchHidden instead of negating a bound function object.

Fixes #1779

Validation

• pnpm --filter vidstack typecheck passed after installing dependencies with pnpm install --frozen-lockfile in the linked worktree.
• Initial typecheck attempt failed because the new worktree had no node_modules and tsgo was missing locally.