feat: add native loop end points

[Kunstderfug]

Description

Add a native end point for loop regions. flutter_soloud already exposes a loop start through loopingStartAt and getLoopPoint / setLoopPoint, but looping always wraps at the natural end of the source. This makes sample-accurate regions inside a longer source impossible.

The end point needs to be enforced by the native mixer while it fills source buffers. Dart-side timers or post-buffer checks can overshoot the requested boundary.

Requirements

• Add an optional loopingEndAt parameter alongside loopingStartAt on the regular, 3D, source, and bus playback APIs.
• Add live-handle getLoopEndPoint and setLoopEndPoint APIs; null clears the end point and restores natural-EOF looping.
• Treat a region as half-open: [loop start, loop end).
• Reject negative start values and end values that are not later than the start.
• Enforce the boundary in native source-frame reads without overshooting a mixer block, including the inaudible-tick path.
• If an end point is beyond the source duration, keep the existing natural-EOF behavior.
• Preserve current first-pass behavior for each playback API and require a source that can seek back to the loop start.
• Cover short regions, exact buffer boundaries, natural EOF fallback, clearing the end point, and invalid ranges with deterministic tests.
• Keep FFI and web/WASM APIs in parity and document the new contract.

Additional Context

The missing end point was discussed while loop-start support was introduced in #43, and the underlying SoLoud limitation was also called out in jarikomppa/soloud#15. The implementation should live at the generic mixer seam so it works consistently across supported source types rather than only for one decoder.