# Phase 10.13.1 — Doctors Parity Gate Optional Property Root Fix

## Root cause
The Phase 10.13 live endpoint returned valid Doctors parity evidence with:
- `functions_checked = 6`
- `max_difference = 0`
- all six parity rows PASS
- Doctors browser rendering PASS 2/2

The PowerShell gate failed while flattening mixed JSON row shapes under `Set-StrictMode -Version 2.0`.
Scalar functions expose `legacy_value` / `new_value` but do not expose `legacy_rows` / `new_rows`.
Ranking functions expose `legacy_rows` / `new_rows` and `ranking_rows` but do not expose scalar value fields.

The old gate accessed optional properties directly and therefore raised:
`The property 'legacy_rows' cannot be found on this object.`

## Root fix
`RUN_DOCTORS_PARITY_GATE.ps1` now uses one strict-mode-safe optional-property reader for every field whose presence depends on comparison type. It also validates the endpoint-reported six checks and exact zero difference without changing any Doctors SQL, repositories, service logic, UI, browser rendering, or Oracle data.

## Scope
- Base release: `10.13.0`
- Target release: `10.13.1`
- Oracle writes: `NONE`
- Business logic changes: `NONE`
- UI changes: `NONE`
- SQL changes: `NONE`