Integrate rough estimate balance

AGENTS.md

@@ -615,6 +615,30 @@ Future sizing will need:
 
 Users must be able to override sizing suggestions.
 
+## Estimate Balance Module (Grobe Leistungsbilanz)
+
+`src/estimate-balance` plus `src/shared/estimate-balance` is a separate module
+for the early-phase power balance by DIN 276 cost group (see
+`docs/estimate-balance.md`). Rules:
+
+- It has its own tables (`estimate_*`) and never writes circuit, board or
+  project-device data; it only reads device rows for the comparison.
+- Writes use the project command mechanism (`estimate-balance.insert/.update/
+  .delete` with complete documents), so revision checks and Undo/Redo apply.
+- The calculation and comparison live in the import-free
+  `src/shared/estimate-balance/estimate-balance.ts` and are shared by API and UI;
+  displayed values use `roundEstimateValue`.
+- It is deliberately not part of named/automatic project snapshots or snapshot
+  restore; those operations leave estimate balances unchanged. Portable project
+  transfer format version 2 includes every estimate balance and remaps balance,
+  group and row ids when duplicating a project. Version 1 transfers remain
+  importable and contain no estimate balances.
+- UI: `/projects/:id/grobbilanz` (Stände) and `/grobbilanz/:balanceId` (editor,
+  `src/frontend/estimate-balance/`). The editor autosaves the whole document
+  debounced (one Undo step per save), keeps invalid raw input local, and on a
+  409 reloads only if the balance itself changed; otherwise it re-saves with
+  the new revision. `request()` throws `ApiRequestError` (status, code).
+
 ## Persistence and Migration Rules
 
 - SQLite is the currently supported database.