FIX pickerfun Python callback

[kroq-gar78]

This restores the old jQuery mechanism from 77a24b5 . I don't know what the pros/cons are of this versus the notify mechanism from legacy (3595800).

Fixes #598 .

[Copilot]

Pull request overview

This PR restores a jQuery-based surface-pick callback path so that pickerfun provided to cortex.webgl.show(...) is invoked from Python when the user clicks on the surface in the WebGL viewer (fixing #598).

Changes:

• Update the /picker Tornado handler to accept voxel coordinates as x,y,z plus vertex and hemi, and call pickerfun(voxel, vertex, hemi).
• Add a default PickPosition.prototype.callback implementation that triggers a GET to /picker on each pick.
• Update show()’s pickerfun docstring / default callback behavior.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 4 comments.

File	Description
cortex/webgl/view.py	Updates picker callback contract and server-side /picker argument parsing/callback invocation.
cortex/webgl/resources/js/facepick.js	Restores a jQuery $.get("/picker?...") request on picks by defining a default callback.

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