Declare tracking lost when the marker leaves the frame (#46)

[kalwalt]

Summary

Fixes #46: after a marker is acquired, isValid() / the getMarker event kept emitting found with a frozen pose indefinitely once the marker left the frame. Two coupled changes in WebARKitTracker.cpp make the tracker declare loss, matching the ArtoolkitX OCVT semantics.

Root cause

WebARKit diverged from ArtoolkitX's tracker state machine in two ways:

1. Track loop gated on _isDetected only. WebARKit resets _isDetected = false every frame and runs detection every frame, so _isDetected means "matched this frame." Once the marker leaves, MatchFeatures fails → _isDetected = false → the optical-flow + template-match path was skipped entirely, so loss was never evaluated. (ArtoolkitX runs that path on a persistent _isDetected with guarded detection, so it re-validates every frame.)
2. _valid not reset on template-match failure. Template matching is the appearance check. When the marker is replaced by a static background, optical flow keeps the points (they don't move → no failure), but the marker template no longer matches → updateTrackableHomography fails. runOpticalFlow's failure path already cleared _valid; RunTemplateMatching's did not — so isValid() stayed true and getMarker kept firing.

ArtoolkitX has no _valid: IsTrackableVisible/GetTrackablePose are gated purely on _isDetected || _isTracking, and RunTemplateMatching failure sets both false.

Changes

• resetTracking(): run the optical-flow/template-match block on _isDetected || _isTracking (was _isDetected), so a tracked-but-not-detected frame still re-validates (and fails → clears state) when the marker is gone.
• RunTemplateMatching() failure path: also set _valid = false (mirrors runOpticalFlow's failure path), so a lost marker propagates to isValid().

Behavior

• Marker present: unchanged — found, stable pose (template matching passes on the real marker; detection re-acquires every frame).
• Marker leaves: within ~1–2 frames, template matching fails → _isDetected/_isTracking/_valid cleared → isValid() false → not found. Content can be hidden by consumers.

Testing

Verified against the live-webcam Teblid example (webarkit/webarkit-testing#37): acquiring the marker shows the AR content; removing it now flips to "Searching…" and hides the content within a frame or two; re-introducing re-acquires; no flicker while the marker is held in view.

Notes / follow-ups

• Independent of the disabled detection guard / downsampling (perf) — that's Restore downsampled feature detection + single-marker detection guard for performance (reconcile with ArtoolkitX OCVT) #44.
• If marginal frames ever cause false-loss flicker on a present marker, add hysteresis / tune the TM_SQDIFF_NORMED < 0.5 template threshold (a robustness layer, not needed in testing).

Fixes #46

🤖 Generated with Claude Code

[kalwalt]

Companion integration PR (submodule bump + rebuilt artifacts + webcam-example verification): webarkit/webarkit-testing#39