fix(flterm): preserve tracked pointer and touch state

[adonm]

Summary

• preserve pointer identity, button, and coordinates across press, motion, release, cancellation, hover, and wheel input
• send wheel events at their actual pointer position and retain fractional wheel deltas
• add opt-in TouchMouseTracking.tapAndScroll: taps reach tracked terminal apps while touch/stylus drags remain local scrolling
• keep TouchMouseTracking.direct as the compatibility default

Why

The previous path could fabricate (0, 0) wheel positions, lose the pressed button, and forward touch drags as held left-button motion, causing applications such as Zellij to select text instead of scrolling.

This is an independent extraction from #101, ported onto current main without the renderer-refactor commits.

Verification

• flutter test with the supported prebuilt native-asset provider: 703 passed
• dart format --output=none --set-exit-if-changed packages/flterm: clean
• dart analyze packages/flterm: only the 3 existing info-level collection-style lints

[adonm]

Addressed by the upstream interaction rewrite. Closing.

Each gap this PR fixed is now covered on main:

• Tracked pointer state: InteractionRegion (bf6ea62) keeps a per-pointer _TrackedPointer (button, buttons, kind, position, tap candidate) across press/motion/release/cancel/hover, and MouseInput.anyButtonPressed landed with the upstream C API regeneration (a3f33fa/…). The nullable-button contract this PR added is the current MouseInput shape.
• Wheel at actual pointer position with fractional deltas: ScrollGestureRegion (6deaece) forwards PointerScrollEvent.localPosition and scrollDelta.
• Touch drags scroll locally, taps reach tracked apps: the interaction region's tap-candidate model (_releaseTrackedPointer sends press+release only when the touch stays within slop) implements the scroll-first behavior; upstream chose always-on rather than the opt-in TouchMouseTracking.tapAndScroll flag this PR proposed.

The tapAndScroll opt-in API is the only piece with no upstream equivalent. If you want it as a separate toggle on the current gesture settings, happy to file a focused follow-up.

[adonm]

Closing as addressed; see the comment above mapping each fix to the upstream commits that cover it.