fix(core): reject transitions after defaults

[Eilen6316]

Short description

Reject transitions added after a default transition so first-match routing cannot silently shadow later rules.

Changes

• Validate default-transition ordering when adding transitions to GraphBuilder.
• Reject duplicate defaults and non-default transitions after an existing default.
• Apply the same ordering check when merging graphs.
• Add regression tests for build-time and add-time validation.

How I tested this

• pytest -q tests/core/test_graph.py tests/core/test_application.py
• pre-commit run --files burr/core/graph.py tests/core/test_graph.py

Notes

The change preserves existing duplicate-default errors and only rejects transitions that would be unreachable.