bug(builder): incremental rebuild drops edges when re-parsing a file

[carlos-alm]

Found during dogfooding v2.4.0

Severity: High
Command: codegraph build . (incremental)

Reproduction

# 1. Clean build
rm -f .codegraph/graph.db
codegraph build . --no-incremental
codegraph stats --json  # 906 edges

# 2. Touch one file and incremental rebuild
echo "// test" >> src/logger.js
codegraph build .
codegraph stats --json  # 860 edges (-46 edges!)

# 3. Full rebuild to confirm
codegraph build . --no-incremental
codegraph stats --json  # 906 edges (restored)

Expected behavior

Incremental rebuild touching one file should produce identical node/edge counts to a full rebuild.

Actual behavior

After touching src/logger.js (appending a comment), 46 edges are lost:

Kind	Full	Incremental	Lost
calls	655	622	-33
imports	115	103	-12
reexports	25	24	-1
contains	111	111	0
Total	906	860	-46

The build output shows "Graph built: 579 nodes, 0 edges" in incremental mode, and the role distribution shifts.

Root cause

When a file is re-parsed incrementally, its old edges are deleted and new edges are re-inserted. However, import resolution in incremental mode may not re-resolve edges for files that import the changed file. The changed file's outgoing import/call edges are restored, but edges from OTHER files that reference the changed file may be dropped.

Suggested fix

During incremental rebuild, after re-parsing changed files, also re-resolve imports for all files that import the changed files (reverse dependency cascade).