* 🐛 Fix nil path content crash by exposing safe public API
Move nil-safety for path segment helpers to the public API layer
(app.common.types.path) rather than the low-level segment namespace.
Add nil-safe wrappers for get-handlers, opposite-index, get-handler-point,
get-handler, handler->node, point-indices, handler-indices, next-node,
append-segment, points->content, closest-point, make-corner-point,
make-curve-point, split-segments, remove-nodes, merge-nodes, join-nodes,
and separate-nodes. Update all frontend callers to use path/ instead of
path.segment/ for these functions, removing the path.segment require
from helpers, drawing, edition, tools, curve, editor and debug.
Replace ad-hoc nil checks with impl/path-data coercion in all public
wrapper functions in app.common.types.path. The path-data helper
already handles nil by returning an empty PathData instance, which
provides uniform nil-safety across all content-accepting functions.
Update the path-get-points-nil-safe test to expect empty collection
instead of nil, matching the new coercion behavior.
* ♻️ Clean up path segment dead code and add missing tests
Remove dead code from segment.cljc: opposite-handler (duplicate of
calculate-opposite-handler) and path-closest-point-accuracy (unused
constant). Make update-handler and calculate-extremities private as
they are only used internally within segment.cljc.
Add missing tests for path/handler-indices, path/closest-point,
path/make-curve-point and path/merge-nodes. Update extremities tests
to use the local reference implementation instead of the now-private
calculate-extremities. Remove tests for deleted/privatized functions.
Add empty-content guard in path/closest-point wrapper to prevent
ArityException when reducing over zero segments.
