refactor(native): consolidate barrel-trace-with-fallback duplication in import_edges.rs

[carlos-alm]

Summary

While fixing #2295 (consolidating TS's duplicate barrel-trace-with-fallback logic onto the shared traceBarrelTarget helper from #2071), found that the Rust native engine has the identical duplication pattern in crates/codegraph-core/src/domain/graph/builder/stages/import_edges.rs — and, unlike TS, had no shared helper at all to consolidate onto.

Three call sites each inlined the same "resolve through a barrel, falling back to the original file/name" pattern independently:

• collect_symbol_lookup_pairs
• emit_named_symbol_rows
• emit_barrel_through_rows

Fix

Extracted trace_barrel_target(ctx, resolved_path, symbol_name) -> BarrelResolution, mirroring TS's traceBarrelTarget (src/domain/graph/builder/stages/resolve-imports.ts) exactly: returns the resolved barrel target when resolved_path is a barrel and resolution succeeds, otherwise falls back to resolved_path/symbol_name unchanged. Rewrote all three call sites to use it.

emit_barrel_through_rows needed a closer look: unlike the other two, it didn't previously have an is_barrel_file guard inline (redundant — the function's own top-level guard already established that) and treated "resolution failed" as an explicit early continue rather than a fallback. After routing through the shared helper (which never returns None, only the fallback identity), that case now flows through the same pre-existing actual_source == resolved_path self-reference check the function already used for a different purpose — same observable outcome, one fewer special case. Verified this is genuinely behavior-preserving by running the new emit_barrel_through_rows tests against both the original and refactored implementations with identical (all-passing) results.

Test plan

• New tests for trace_barrel_target directly: falls back to identity for a non-barrel file, resolves through a barrel to the declaring file, falls back to identity when barrel resolution finds nothing
• New tests for emit_barrel_through_rows (previously had no dedicated unit test — only exercised indirectly through the emit_edges_for_import orchestrator): emits an edge to the actual definition file, emits nothing when barrel resolution finds no target, skips a barrel that resolves back to itself
• Revert-verified behavioral equivalence: temporarily restored the original three inline implementations (keeping the new tests) and confirmed the emit_barrel_through_rows tests still pass unchanged against the pre-refactor code — proving the refactor doesn't alter observable behavior
• cargo test --lib (full suite) — 1094 passed (up from 1088, +6 new)
• cargo clippy --lib -- -D warnings — clean
• Rebuilt the native .node addon and ran tests/integration/build-parity.test.ts (native vs WASM cross-engine parity, including the monorepo-workspace fixture) — 12/12 passed
• npm test (full suite) — 339 files, 5447 passed
• npx tsc --noEmit — clean (no TS files changed by this PR)

Closes #2461

[greptile-apps]

Greptile Summary

The PR consolidates duplicated native barrel-resolution fallback logic into trace_barrel_target while preserving parity with the TypeScript implementation.

• Reuses the helper for symbol lookup, named-symbol edge emission, and barrel-through edge emission.
• Adds focused coverage for successful resolution, identity fallback, unresolved exports, and self-referencing barrels.

Confidence Score: 5/5

The PR appears safe to merge with no actionable defects identified.

The shared helper preserves each call site's previous fallback semantics, and unresolved barrel exports still produce no through-edge because the existing self-reference check discards the identity fallback.

Important Files Changed

Filename	Overview
crates/codegraph-core/src/domain/graph/builder/stages/import_edges.rs	Extracts a shared barrel-target resolution helper, updates three equivalent call sites, and adds regression tests without introducing an actionable behavioral change.

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