fix: derive edge-only DataFrame nodes correctly

[j6k4m8]

Summary

• derive edge-only nodes from the unique source/target union
• check endpoint values rather than pandas row indexes for membership
• report accurate node counts without a separate node dataframe
• cover custom columns, stable enumeration, metadata mode, and missing nodes

[codspeed-hq]

Merging this PR will not alter performance

✅ 17 untouched benchmarks

---

_{Comparing fix/dataframe-edge-only-nodes-76 (615ac49) with master (228fa6d)}