Split graph fact storage modules

[yvette-carlisle]

Summary

• Split elf-storage graph fact helpers into normal Rust child modules:
  • graph/fact/write.rs for insert/upsert fact writes and evidence links
  • graph/fact/query.rs for active fact reads
  • graph/fact/supersession.rs for active fact supersession
• Kept graph/fact.rs as the private facade that re-exports the same four functions.
• Preserved the existing public elf_storage::graph::* surface through graph.rs.

Validation

• cargo check -p elf-storage --all-targets --all-features
• cargo make fmt-rust-check && cargo make check-rust && cargo make lint-vstyle && git diff --check
• cargo make lint-rust && cargo make test-rust
• cargo make check
• Docker-backed cargo make test-rust-integration
  • 92 passed, 370 skipped
  • Includes elf-storage::graph_memory and graph ingestion acceptance coverage.
• cargo make check-trace-gate
  • Initial run used a non-default Postgres port while the fixture config points at 127.0.0.1:5432.
  • Re-ran with fixture-default Postgres on 5432; passed.

Review Notes

• Scout selected this split because graph/fact.rs had independent write/query/supersession responsibilities and the existing graph/predicate.rs already uses the same facade pattern.
• Skeptic found no blockers; the main risk was staging the new child modules with the facade, which this PR does.