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.

docs/SPECIFICATION.md

@@ -37,7 +37,7 @@ Silo treats **part numbering schemas as configuration, not code**. Multiple numb
                               ▼
 ┌─────────────────────────────────────────────────────────────┐
 │                   Silo Server (silod)                        │
-│  - REST API (74 endpoints)                                  │
+│  - REST API (76 endpoints)                                  │
 │  - Authentication (local, LDAP, OIDC)                       │
 │  - Schema parsing and validation                            │
 │  - Part number generation engine                            │
@@ -469,6 +469,53 @@ child_revision: null  # means "latest"
 
 Assembly metadata YAML controls default behavior per assembly.
 
+### 8.4 Flat BOM and Assembly Costing
+
+Two endpoints provide procurement- and manufacturing-oriented views of the BOM:
+
+**Flat BOM** (`GET /api/items/{partNumber}/bom/flat`) walks the full assembly tree and returns a consolidated list of **leaf parts only** (parts with no BOM children). Quantities are multiplied through each nesting level and duplicate parts are summed.
+
+```
+Assembly A (qty 1)
+├── Sub-assembly B (qty 2)
+│   ├── Part X (qty 3)   → total 6
+│   └── Part Y (qty 1)   → total 2
+└── Part Z (qty 4)       → total 4
+```
+
+Response:
+
+```json
+{
+  "part_number": "A",
+  "flat_bom": [
+    { "part_number": "X", "description": "...", "total_quantity": 6 },
+    { "part_number": "Y", "description": "...", "total_quantity": 2 },
+    { "part_number": "Z", "description": "...", "total_quantity": 4 }
+  ]
+}
+```
+
+**Assembly Cost** (`GET /api/items/{partNumber}/bom/cost`) builds on the flat BOM and multiplies each leaf's `total_quantity` by its `standard_cost` to produce per-line extended costs and a total assembly cost.
+
+```json
+{
+  "part_number": "A",
+  "total_cost": 124.50,
+  "cost_breakdown": [
+    { "part_number": "X", "total_quantity": 6, "unit_cost": 10.00, "extended_cost": 60.00 },
+    { "part_number": "Y", "total_quantity": 2, "unit_cost": 7.25, "extended_cost": 14.50 },
+    { "part_number": "Z", "total_quantity": 4, "unit_cost": 12.50, "extended_cost": 50.00 }
+  ]
+}
+```
+
+Both endpoints detect BOM cycles and return **HTTP 409** with the offending path:
+
+```json
+{ "error": "cycle_detected", "detail": "BOM cycle detected: A → B → A" }
+```
+
 ---
 
 ## 9. Physical Inventory
@@ -619,6 +666,8 @@ POST   /api/items/{partNumber}/file                         # Upload file [edito
 # BOM
 GET    /api/items/{partNumber}/bom                          # List direct children
 GET    /api/items/{partNumber}/bom/expanded                 # Multi-level BOM (recursive)
+GET    /api/items/{partNumber}/bom/flat                     # Flattened BOM (leaf parts, rolled-up quantities)
+GET    /api/items/{partNumber}/bom/cost                     # Assembly cost roll-up
 GET    /api/items/{partNumber}/bom/where-used               # Where-used (parent lookup)
 GET    /api/items/{partNumber}/bom/export.csv               # Export BOM as CSV
 GET    /api/items/{partNumber}/bom/export.ods               # Export BOM as ODS
@@ -673,11 +722,13 @@ POST   /api/inventory/{partNumber}/move
 - [x] YAML schema parser for part numbering
 - [x] Part number generation engine
 - [x] CLI tool (`cmd/silo`)
-- [x] API server (`cmd/silod`) with 74 endpoints
+- [x] API server (`cmd/silod`) with 76 endpoints
 - [x] MinIO integration for file storage with versioning
 - [x] BOM relationships (component, alternate, reference)
 - [x] Multi-level BOM (recursive expansion with configurable depth)
 - [x] Where-used queries (reverse parent lookup)
+- [x] Flat BOM flattening with quantity roll-up and cycle detection
+- [x] Assembly cost roll-up using standard_cost
 - [x] BOM CSV and ODS export/import
 - [x] Reference designator tracking
 - [x] Revision history (append-only) with rollback and comparison