Add static image example with Teblid tracker (#30)

[kalwalt]

Summary

Implements #30: a static-image AR example that feeds a fixed JPEG into the Teblid tracker instead of a webcam stream, so the tracker can be exercised without camera access (useful for debugging and CI). Structure mirrors threejs_ES6_example.html, reusing worker_threejs.js and the existing data/pinball.jpg marker unchanged.

The marker tracks successfully. The 3D sphere is not yet visible due to a pre-existing pose-convention issue, filed separately as #31 (out of scope here).

What's included

File	Purpose
examples/threejs_teblid_static_image_ES6_example.html	Page with a hidden <img> input (no initCamera); load event starts tracking
examples/threejs_static_image_worker_ES6.js	Client worker: marker decode + full-res RGBA processing + warmup/ping-pong
examples/data/pinball-demo.jpg	Test image containing the marker
docs/design-static-image-teblid-example.md	Design notes, decision log, and bug analysis

worker_threejs.js, data/pinball.jpg, and the CSS/JS assets are reused without modification.

Bugs found & fixed (to make tracking actually work)

All fixes live in the new client worker — worker_threejs.js is untouched:

1. Marker was passed as raw JPEG bytes. initTrackerGray() treats its buffer as raw pixels (no JPEG decode), so the ~764 KB compressed file was read as a 13 MB RGBA reference → garbage marker, matching always failed. Now decoded to RGBA pixels via a plain 2D canvas (no GrayScale/WebGL class, per request).
2. Init/process dimension mismatch. The WASM frame buffer is sized videoWidth × videoHeight, but the original threejs worker sent a downscaled frame. Now processes the image at full resolution to match.
3. First-frame perspectiveTransform crash. When a marker is detected on frame 0 (_frameCount == 0), the tracker calls GetTrackedFeaturesWarped() before the tracked-feature set is populated → OpenCV assert (scn+1 == m.cols). Worked around with a blank warmup frame; frames are fed via worker ping-pong (throttles heavy full-res processing) and stop once tracked.

Review checklist

• worker_threejs.js and data/pinball.jpg are genuinely unchanged
• Marker decode path uses only a 2D canvas (no GrayScale class)
• Blank warmup frame + ping-pong logic reads cleanly
• Comfortable including the design doc in docs/ (vs. PR description only)
• pinball-demo.jpg is the intended test asset

Risk assessment

• Low risk / additive. New files only; no existing example or shared worker is modified, so no regression surface for current examples.
• The full-res frame (2000×1500) is heavy, but ping-pong + stop-after-found keeps it to a few frames on a static image.

Test plan

• Serve examples/ over HTTP and open threejs_teblid_static_image_ES6_example.html
• Console shows the blank warmup frame ("No features…"), then Marker tracked ! Num. matches : N
• Loader overlay disappears
• Confirm existing webcam examples are unaffected

Known follow-up

• 3D pose renders behind the camera (CV vs GL convention); matrixGL_RH throws #31 — 3D pose renders behind the camera (OpenCV +Z vs OpenGL −Z; matrixGL_RH throws). Fix lives in the WebARKitLib submodule + WASM rebuild.

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