Thin public wrapper around the internal _shapeAt. Returns the top-
most shape descriptor under pt (uuid, kind, geometry, image_idx /
image_id, style, metadata) or null. Strip handles take
pt = {imageIdx, x, y} in source-pixel space; canvas handles take
pt = {x, y} in canvas-pixel space.

Consumers wiring custom tap-zone navigation that bypasses fresco's
tap-bus (left/right page-flip thirds, sidebars, mini-maps) can now
defer to etcher's per-kind hit-test instead of re-implementing one
per shape kind.

Pairs with fresco 0.6.3's suppress-tap fix — most consumers don't
need this if they rely on data-fresco-suppress-tap, but custom tap
handlers do.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>