test: add test coverage for storage.ts (saveGraph / loadGraph)

[TechPMRyan]

Summary

src/graph/storage.ts has zero test coverage. It contains two functions — saveGraph and loadGraph — that are part of the critical path between a completed scan and a usable graph. Neither is tested.

Additionally, loadGraph has a silent failure: it catches a JSON parse error and returns null with no log and no context. A missing file and a corrupted file are indistinguishable to the caller. That is a bug in the contract.

What needs to happen

• Write a test file for storage.ts
• Test loadGraph returns null on a missing file path
• Test loadGraph returns null (or throws with context) on a corrupted JSON file — these two states should be distinguishable
• Test saveGraph writes a file that loadGraph can read back correctly (round-trip test)
• Test saveGraph behavior when the target path is not writable

Why this matters

The storage layer is the only persistence mechanism for scan output. Silent failures here mean a user runs a scan, gets no graph, and has no idea why. That is the worst possible failure mode for a tool whose core value is showing you a graph.

Acceptance criteria

• loadGraph on a missing file: returns null, no error thrown
• loadGraph on corrupted JSON: distinguishable from missing file (log or structured error)
• Round-trip test: saveGraph then loadGraph returns the original graph
• saveGraph error handling on unwritable path is explicit and surfaces to the caller