Track Architectural Stability Across Snapshots

[akhundMurad]

Pacta currently captures architecture snapshots but does not quantify how stable the architecture is over time.
This issue proposes introducing an Architectural Stability Index that measures how much of the system remains unchanged between snapshots.

The goal is to provide a simple, explainable signal that highlights volatile areas and long-term architectural churn.

Proposed definition

stability = unchanged_components / total_components

A component is considered unchanged if its identity and dependency set remain the same between snapshots.

Derived metrics

• Stability percentage per snapshot
• Architectural churn percentage
• Identification of volatile components (frequently changing)

Example insight

The auth module changed in 7 of the last 8 snapshots, indicating an unstable core component.

Acceptance Criteria

• Architectural stability is computed between consecutive snapshots
• Stability and churn values are persisted as snapshot-level metrics
• Volatile components can be identified based on change frequency
• Metrics are accessible via existing reporting mechanisms (CLI / JSON)
• Metric definition is documented