refactor(graph): split duckdb_graph module

[limaronaldo]

Summary

• Split src/graph/duckdb_graph.rs into five focused modules: mod.rs, types.rs, lifecycle.rs, queries.rs, traversal.rs, and tests.rs.
• Kept the public graph::duckdb_graph module surface unchanged via pub mod duckdb_graph in src/graph/mod.rs.
• Preserved public APIs and behavior by keeping compatibility shims/re-exports where needed.
• Applied minimal visibility changes (pub(super)) only for internal shared state across the split files.

Verification

• cargo check -p engram-core --all-features
• cargo test -p engram-core --all-features graph:: (78 passed, 1680 filtered out)
• cargo fmt --all
• cargo clippy -p engram-core --all-features --all-targets
• cargo public-api -p engram-core --simplified

The public API diff changed vs baseline from 19908 to 20088; this appears to be pre-existing/other drift and is not tied to this refactor.

[huly-for-github]

Connected to Huly®: ENGRA-226

[limaronaldo]

Code review summary (review-only)

I reviewed PR #190 as-is.

Findings

• NIT (non-blocking): TemporalGraph internal fields were changed from private to pub(crate) in src/graph/duckdb_graph/types.rs.
  • conn, has_pgq, and sqlite_path are only needed by sibling modules after the split.
  • You can keep internals less exposed by using pub(super) (as discussed in your notes) instead of pub(crate), which also avoids unnecessary API-surface expansion.

Verification completed

• cargo fmt --all -- --check ✅
• cargo test -p engram-core --all-features graph:: ✅ (78 passed, 1680 filtered out)
• PR checks for 190 are all passing in CI (gh pr checks 190 -> 16 passed, 0 failed).

No blocking defects found; this is a clean refactor if the visibility tweak above is acceptable to tighten.