fix: preserve igraph collision attributes across round trips

[lmeyerov]

Summary

• Restore igraph source/target collision attributes when endpoint bindings use custom names.
• Keep default-binding collision behavior and retain attributes under unique fallback names.
• Reserve temporary and final-binding names through one allocator so chained compute_igraph calls remain schema-stable and avoid duplicate-column crashes.

Boundary Coverage

• Positive controls: source/target collision attributes, occupied final-binding fallbacks, repeated round trips, large nullable values, direct and GFQL/wire invocation, repeated chain enrichment, and Polars bridge retention.
• Exact mixed final-binding siblings run in the CPU/igraph suite: source attribute collision with destination __attr_source_2__, and target attribute collision with source __attr_target_2__.
• Negative/control evidence reuses focused immutability and repeated-chain cases; no unrelated broad campaign.

Evidence

• Reproduced first-call schema leakage and second-call duplicate-column failure at master d220c9cdc8….
• Exact repaired head: 76786f4eb5c356ff7bf1adb82a8a194bfb7e79d6.
• Focused igraph suite: 48 passed on host and 48 passed, 5 subtests passed in the 26.02 GPU image.
• Polars bridge remains green: 20 passed, 4 skipped. Ruff is clean.
• Full-tree cuDF bridge proof on dgx-spark (graphistry/test-rapids-official:26.02-gfql, cuDF 26.02.01): validates native-cuDF bridge stability with s/d bindings and generated-looking attribute names, plus chained CPU igraph enrichment, from an exact git archive; imports resolve to /work/graphistry/__init__.py. Receipt: worker/research/round-002/S4-2204-cudf-mixed-binding-76786f4e.txt.
• Whole-tree archive SHA-256: 1f581652750db837d8db99e9df70c927aab05d721494eb33ec4a186695fb6a17.
• Final evidence: worker/research/round-002/repair-final-evidence-2026-10-20.md.
• igraph is CPU-only; this validates the cuDF engine bridge, not GPU igraph execution.

Closes #2204