docs(prd): add allocated layer design addendum
Open-source Excel-native EPM and consolidation for SAP & Dynamics
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Originally created by: grynn-in
Closes open design questions for PRD-ALLOCATED-LAYER: FCTB boundary, alloc_fully_loaded_tb grain/join logic, run semantics, budget recon mapping, build order, and a decision log. Link from parent PRD and index.
Ticket changed by: grynn-in