feat(g.Point): add chooseClosestIndex() and string support in update()

[kumilingus]

Summary

• g.Point#chooseClosestIndex(points) — returns the index of the closest point in points, -1 for an empty array. Useful when the caller needs its own anchor object back rather than a g.Point copy (the chooseClosest() contract). chooseClosest() is now a thin wrapper over it.
• Zero per-candidate allocation — the loop reuses one scratch Point via update() instead of new Point(points[i]) for every item.
• g.Point#update() accepts strings — '10 20' / '10@20' are parsed the same way the constructor does (shared parsePointString()). Previously update('10 20') silently stored the string as x and 0 as y. Typing widened from PlainPoint to PointInit to match the constructor and runtime.
• fixed-connection-points story — hand-rolled closest-anchor loop replaced with chooseClosest() (the story only reads x/y, so the copy is fine); the | null return makes the empty-anchors case explicit and falls back to the drop point instead of indexing past the array.

Changesets: two minor entries, one for the new method and one for the update() string support (new PointInit overload in the public typings).

The same hand-rolled loop lives in joint-demos/fixed-connection-points. Like the story, it only reads x/y from the result, so it can switch to the already-released chooseClosest() independently of this PR. chooseClosestIndex() is for callers that need their original anchor object back (identity, or anchors carrying extra data such as an id or side).

Test plan

• karma:geometry — 353/353 (new tests: chooseClosestIndex incl. empty / single / g.Point / string / partial { x } inputs and tie → first index; update() with string arg). Each new test was watched failing first.
• grunt test:ts (core typings)
• joint-react: eslint on the story; tsc --noEmit reports no errors in touched files
• eslint on all touched core files

🤖 Generated with Claude Code