INST-5a · Persist the reviewer's initial classification as a durable field

> Item derived from the fact-checking cycle analysis (**2026-07-03, v2**). Split from INST-5 (#2495). Companion proposal **ENG-2** — its strongest point.

### Finding (evidence)
Cross → final adoption (old 98.7%) is **unauditable** — `report.classification` equals the initial value. The reviewer's initial classification already exists **transiently** in ReviewTask history.

### Action
Persist the reviewer's **initial classification** as a durable field. This is the smallest change with the largest audit payoff, and guarantees the 98.7% storage artifact can never recur.

### Impact
The three-way initial→cross→final reconstruction becomes a first-class query.

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- **Priority:** 🔴 High · **Effort:** P (quick win)
- **Parent:** #2495 · **Epic:** #2489