Harden traversal and A-star behavior

[davidkpiano]

Summary

• keep active BFS, DFS, and postorder traversal structure stable across graph mutations without bypassing the cached CSR fast path
• add lazy multi-source, directional, radius-limited genPostorder and make getPostorder its eager counterpart
• reject non-finite A-star heuristic values before they enter heap ordering

Why

Active traversals retained a CSR snapshot but read nodes from the live array, so node deletion could produce wrong nodes or undefined. Postorder did not support the traversal options shared by BFS and DFS. Non-finite A-star priorities could return a suboptimal path instead of rejecting invalid input.

Validation

• pnpm verify
• 82 test files, 1,852 tests passed
• type checks, generated schemas, API conventions, build, publint, and package smoke test passed

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 2 additional findings.