Allow editing transition labels by double-clicking

[cobalt-arc-dev]

Fixes #39.

Summary:

• Add hit testing for rendered transition label text, not just transition arrowheads.
• Return the existing transition edit target when a label is double-clicked, so the current label input flow opens unchanged.
• Match the rendered label position for straight, arced, detached, and self transitions.

Validation:

• node --check site/js/render.js
• find site/js -name '*.js' -print0 | xargs -0 -n1 node --check
• git diff --check
• Node VM harness covering normal transition label hit testing, self-transition label hit testing, and state hit testing.