feat(architecture): aggregate module-to-package dependency edges on the graph #507

Problem

On a real repository (PARTHA itself: 343 nodes, 13,834 relationships), 11,225 of the edges are module-to-package dependency edges. They dominate the graph and are the reason large repositories fall back to the limited-edge mode introduced in #502.

Idea

Represent dependencies at package level by default: one edge per (layer or module group, package) with a count, expandable to the individual modules on demand. An edge-type filter (hide or show dependency edges) would be a smaller first step.

Acceptance criteria

• On the repository above, the default graph shows meaningfully fewer edges without hiding that dependencies exist.
• Counts are accurate and traceable to the underlying relationships (the project does not guess).
• Graphs under the threshold are unchanged unless the user opts in.
• Unit tests for the aggregation; no regression in the existing Playwright specs.

Pointers

apps/frontend/src/features/architecture/layout.ts, components/ArchWorkspace.tsx, RelationshipPanel.tsx. Edge type values seen: dependency, import, calls, api-call.