docs: document flat BOM and assembly costing endpoints

Add section 8.4 to SPECIFICATION.md describing the flat BOM flattening
and assembly cost roll-up endpoints with example request/response JSON.

- GET /api/items/{pn}/bom/flat — consolidated leaf parts with
  rolled-up quantities and cycle detection
- GET /api/items/{pn}/bom/cost — per-line extended costs and total
  assembly cost using standard_cost

Update endpoint count from 74 to 76 in SPECIFICATION.md and README.md.
Add checklist entries for flat BOM and assembly costing features.

README.md

@@ -8,7 +8,7 @@ Kindred Silo is an R&D-oriented item database with:
 
 - **Configurable part number generation** via YAML schemas
 - **Revision tracking** with append-only history, rollback, comparison, and status labels
-- **BOM management** with multi-level expansion, where-used queries, CSV/ODS export
+- **BOM management** with multi-level expansion, flat BOM flattening, assembly costing, where-used queries, CSV/ODS export
 - **Authentication** with local (bcrypt), LDAP/FreeIPA, and OIDC/Keycloak backends
 - **Role-based access control** (admin > editor > viewer) with API tokens and sessions
 - **ODS import/export** for items, BOMs, and project sheets
@@ -25,7 +25,7 @@ silo/
 │   ├── silo/          # CLI tool
 │   └── silod/         # API server
 ├── internal/
-│   ├── api/           # HTTP handlers, routes, templates (74 endpoints)
+│   ├── api/           # HTTP handlers, routes, templates (76 endpoints)
 │   ├── auth/          # Authentication (local, LDAP, OIDC)
 │   ├── config/        # Configuration loading
 │   ├── db/            # PostgreSQL repositories