Structured Output Schema Hardener

Harden this output schema so a model cannot produce something that parses but is wrong.

For each field, check and fix:
- Is the type as narrow as it can be? Prefer an enum over a string, an integer over a number, a bounded array over an unbounded one.
- Can the model omit it? Should it be able to?
- Is there a sensible value the model will pick when it does not know? If so, add an explicit unknown option so the model does not fabricate.
- Does the field name alone tell the model what goes in it, with no schema description?
- Are there two fields a model could plausibly swap?

Then give me:
1. The hardened schema
2. The three most likely ways a model still produces valid-but-wrong output
3. A validation rule for each, to run after parsing

Schema:
[PASTE JSON SCHEMA]