[Feature] Incremental CPG update for javasrc2cpg (file-level re-indexing without full rebuild)

[CheolHoJung]

Is your feature request related to a problem? Please describe.

When using Joern as a persistent analysis server, re-indexing a project after a source file changes requires a full importCode rebuild. For large Java projects this takes several minutes (~10 min in our case), making interactive workflows impractical.

Describe the solution you'd like

An IncrementalCpgUpdater utility that re-indexes only the changed files in an existing CPG:

IncrementalCpgUpdater.updateFiles(cpg, project.inputPath, List("/path/to/Changed.java"))

Internally this would:

1. Delete CPG nodes for the changed files (File nodes + NamespaceBlock AST subtrees)
2. Re-parse only those files via AstCreationPass(sourcesOverride = Some(changedFiles))
3. Re-run the standard post-passes (OuterClassRefPass, TypeNodePass, TypeInferencePass, AstLinkerPass, ContainsEdgePass, StaticCallLinker, etc.)

We have a working prototype for javasrc2cpg with 8 unit tests passing (method add/remove, caller/callee edge add/remove). Performance: ~350ms per file vs ~10min for a full rebuild (~1700× speedup).

Describe alternatives you've considered

• Full importCode rebuild on every change — too slow for interactive use
• Watching only specific node types — incomplete, misses call graph edges

Additional context

This is currently javasrc2cpg-specific. Happy to open a PR if the approach looks reasonable — would appreciate guidance on whether the HTTP endpoint (PUT /v1/cpg/:projectName/files) belongs in joern-cli or is out of scope.