[Feature]: Add structured JSON output

[harryswift01]

Feature Request

Problem / Motivation

Users should be able to run SIST once and save the important results in a reusable, machine-readable format.

The current text output is not suitable for reliable downstream processing.

Proposed Solution

Add structured internal result objects and write them using the agreed JSON schema.

The existing text output should remain available for compatibility.

Proposed Work

• Add internal objects for intermediate and final results.
• Capture calculation metadata, warnings, and execution status.
• Write the result to JSON.
• Add configuration for the output path.
• Validate generated files against the schema.
• Represent partial and failed calculations clearly.
• Preserve the existing text output.
• Add regression and end-to-end tests.

Done When

• A normal run produces a valid JSON result file.
• Results agree with the baseline within accepted tolerances.
• Metadata and warnings are included.
• Partial and failed runs are represented clearly.
• Existing text output continues to work.
• The implementation is covered by automated tests.