Add a fully local calibration workflow (no Modal/HF dependencies)

[baogorek]

Problem

It's too easy to accidentally invoke Modal or Hugging Face downloads when trying to work locally on calibration. For example, make build-matrices goes to Modal, and make calibrate has a data prerequisite that can trigger HF downloads. During debugging, this breaks the closed chain of local inputs → local computation → local outputs, making it very hard to trace issues.

This came up while debugging the ACA PTC county mismatch in #538 — the local/remote boundary confusion cost time and made the investigation harder than it needed to be.

What's needed

A clearly documented, fully local path through the calibration pipeline where:

• Data inputs come from local files only (no HF downloads)
• Matrix building runs locally (no Modal)
• Calibration runs on local CPU
• H5 building uses local outputs

Concretely:

• Add build-matrices-local Makefile target (or similar)
• Ensure local targets don't have prerequisites that silently reach out to remote services
• Document the local workflow in the Makefile or README

Context

See PR #538 (calibration-pipeline-improvements branch) for the current state of the Makefile targets and the debugging context that surfaced this issue.