[Feature]: Add loading and validation for saved SIST results

[harryswift01]

Feature Request

Problem / Motivation

Saved results need a supported loading interface so that every downstream tool does not have to implement its own JSON parsing and schema checks.

Proposed Solution

Add a result loader that validates saved files and reconstructs supported SIST result objects.

Proposed Work

• Load structured result files.
• Validate the schema version and required fields.
• Reconstruct result objects.
• Preserve arrays, units, indexing, metadata, and warnings.
• Return clear errors for malformed or unsupported files.
• Define compatibility behaviour between schema versions.
• Add write-and-load round-trip tests.

Done When

• Valid results can be loaded without rerunning SIST.
• Loaded values match the written values.
• Arrays and indexing information are preserved.
• Metadata and warnings are preserved.
• Invalid or unsupported files produce clear errors.
• Round-trip tests cover representative results.