Precompute reverse dependency graph for diff-mode change-impact analysis

[alxxjohn]

Motivation

• The diff-mode change-impact pass previously rebuilt the reverse dependency adjacency map for each changed module, producing O(C*E) work and repeated allocations where C is changed modules and E is import edges.
• This exposed an algorithmic DoS risk when scanning large attacker-controlled diffs.
• The fix precomputes the reverse graph once per target graph while preserving existing artifact and finding behavior.

Description

• Add reverseDependencies() to moduleGraph to build a reverse adjacency map[string][]string once after graph construction.
• Compute reverse := item.graph.reverseDependencies() once per targetModuleGraph in changeImpactFindings and reuse it for all changed files.
• Change changeImpactEntry to accept reverse map[string][]string and use support.TransitiveDependents(reverse, module) to derive dependents.
• Preserve existing sampling, thresholding, artifact emission, and finding messages so functionality and outputs remain unchanged.

Testing

• Ran go test ./internal/codeguard/checks/design ./tests/checks and the targeted tests completed successfully.
• Ran go test ./... and the full test suite completed successfully.
• Ran gofmt and git diff --check to ensure formatting and no diff-check issues and they passed.

---

Codex Task