INST-5b · Store all three classifications + materialized agreement booleans

> Item derived from the fact-checking cycle analysis (**2026-07-03, v2**). Split from INST-5 (#2495).

### Action
Store the three classifications separately (initial / crossCheck / final) + materialized booleans:
- `agreement.initialVsCross`
- `agreement.crossVsFinal`

### Impact
Inter-rater and adoption become a simple `$avg`. Builds on INST-5a (#2495 → 5a) by adding the cross/final fields and the agreement flags.

### Note (D2)
Do **not** cite the 44.4% "adoption in divergent cases" figure as fact until 5a/5b ship — it depends on an independent `final` that does not exist today. Citable now: only the 84.5% (N=187) initial↔cross concordance.

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- **Priority:** 🟡 Medium · **Effort:** M
- **Parent:** #2495 · **Epic:** #2489