FAT workstation convergence: architecture freeze and canonical shell staging

[masarray]

Purpose

Stage the professional Engineering-integrated FAT convergence without destabilizing PR #290's current known-good baseline.

This PR is intentionally stacked on feat/native-fat-workspace (PR #290), not main. It is the protected staging lane for the architecture cleanup and field gates.

M0 completed in this head

• freezes the FAT Engineering workstation ownership and acceptance contract in docs/FAT_ENGINEERING_WORKSTATION_CONTRACT.md
• preserves Engineering as the single parsed SCL/static-DataSet/live-value authority
• preserves strict duplicate-live-reference preflight and the automatic static-mode manual-alias retirement regression
• removes MainWindow.ProductionFatNavigationParity.cs, eliminating the second ModuleInitializer / ApplicationIdle seven-slot navigation correction
• updates regression coverage so that duplicate late navigation parity cannot be silently reintroduced

Protected behavior

No production FAT capture/evidence semantics, command safety sequencing, MMS/report acquisition, GOOSE, RCB, or persistence behavior is changed in M0.

Next gates

• M1: canonical seven-destination shell/navigation
• M2: permanent FAT surface; remove launcher/black transition from primary Engineering FAT flow
• M3+: selected-IED view context, latched capture context, report-preview parity

Do not merge this stacked PR into PR #290 until CI and the relevant field gate are green.