bug: incremental builds produce fewer edges than full builds (3.3% gap)

[carlos-alm]

Found during dogfooding v3.3.0

Severity: Medium
Command: `codegraph build` (incremental vs full)

Reproduction

```bash

1. Full rebuild

codegraph build --no-incremental
codegraph stats --json # record edge count

2. Touch one file, incremental rebuild

echo "// test" >> src/shared/constants.js
codegraph build
codegraph stats --json # record edge count

3. Compare

```

Expected behavior

Incremental and full builds should produce identical node and edge counts when analyzing the same source tree.

Actual behavior

Metric	Incremental	Full Rebuild	Delta
Nodes	5402	5402	0
Edges	11,444	11,834	-390 (3.3%)
Dead exports warning	104 unused	86 unused	+18

The incremental pipeline misses ~390 edges compared to a full rebuild. The higher dead export count in incremental mode suggests some cross-file edges (likely dataflow or call edges) are not being rebuilt for files outside the reverse-dependency set.

Root cause

Likely the reverse-dependency set computed during incremental builds doesn't capture all files that need edge re-evaluation. Files that consume dataflow edges or have indirect call relationships through the changed file may not be included in the re-parse set.

Suggested fix

Compare the edge sets between incremental and full builds to identify which edge kinds are missing (calls, dataflow, contains, etc.) and which files they belong to. The reverse-dependency computation in the incremental pipeline may need to consider additional edge types.