ar_measure_flutter

Fix AR snapshot: retry transient PixelCopy failures, reply once

Fixes ARSessionManager.snapshot() on Android, which could fail on the very frame a caller asked for it and reported nothing useful when it did. Callers compositing the snapshot with their own overlay were left with the overlay alone on an empty background.

• Retries PixelCopy on ERROR_SOURCE_NO_DATA and ERROR_TIMEOUT — the surface has simply not produced a frame yet, which is likely when the snapshot is requested from a tap handler. Non-transient failures still return immediately.
• Reports the actual PixelCopy result (ERROR_SOURCE_INVALID, ERROR_SOURCE_NO_DATA, …) instead of a generic "Failed to capture snapshot", so a failure can be diagnosed from logs.
• Replies to the method channel exactly once. Both the copy listener and the surrounding catch could answer, which crashed the engine with "Reply already submitted".
• Fails with a clear message when the AR view has not been laid out, rather than throwing out of Bitmap.createBitmap(0, 0, …).
• Encodes the PNG off the main thread and recycles the bitmap.

First release of ar_measure_flutter, forked from ar_flutter_plugin_2 0.0.3.

Everything from upstream is retained unchanged. Added, to make aim-a-reticle measuring flows possible:

• ARSessionManager.hitTestAtScreenPosition(x, y) — hit test any screen point, not just a tap.
• ARSessionManager.setFrameUpdates(...) + onFrameUpdate — throttled per-frame stream carrying the reticle hit together with the camera view/projection matrices and viewport, driven from the existing native frame loop (one message per sample, no per-frame round-trips).
• ARFrameUpdate with project(Vector3) -> Offset?, for drawing Flutter overlays that stay anchored to world positions as the camera moves.

Currently implemented on Android (ARCore); the iOS bridge is not yet wired.