Evaluate and implement optional persistent cache support

[Matographo]

Summary

Design and, if justified, implement a disk-backed cache for repeated CLI executions.

Why

Persistent caching may provide significant cold-start wins, but only if the invalidation model remains correct and predictable.

Acceptance Criteria

• The design decision is documented
• If implemented, persistent caching is optional and deterministic
• Cache invalidation rules are clear and tested

Dependencies

• Depends on Implement in-memory caching for parsed templates and include dependencies #24