Prevent default transitions from shadowing later edges

[Eilen6316]

GraphBuilder currently allows adding conditional transitions after a default transition. Burr uses first-match transition semantics, so any edge added after a default edge is silently unreachable.

When adding transitions, validate that:

• no edge can be added after an existing default edge;
• a default transition is the final edge for a node;
• multiple default transitions are rejected.

This issue is tracked by PR #906.

The behavior change should be called out in the release notes.