feat: LibreOffice Calc extension, ODS library, AI description, audit design

Calc extension (pkg/calc/):
- Python UNO ProtocolHandler with 8 toolbar commands
- SiloClient HTTP client adapted from FreeCAD workbench
- Pull BOM/Project: populates sheets with 28-col format, hidden property
  columns, row hash tracking, auto project tagging
- Push: row classification, create/update items, conflict detection
- Completion wizard: 3-step category/description/fields with PN conflict
  resolution dialog
- OpenRouter AI integration: generate standardized descriptions from seller
  text, configurable model/instructions, review dialog
- Settings: JSON persistence, env var fallbacks, OpenRouter fields
- 31 unit tests (no UNO/network required)

Go ODS library (internal/ods/):
- Pure Go ODS read/write (ZIP of XML, no headless LibreOffice)
- Writer, reader, 10 round-trip tests

Server ODS endpoints (internal/api/ods.go):
- GET /api/items/export.ods, template.ods, POST import.ods
- GET /api/items/{pn}/bom/export.ods
- GET /api/projects/{code}/sheet.ods
- POST /api/sheets/diff

Documentation:
- docs/CALC_EXTENSION.md: extension progress report
- docs/COMPONENT_AUDIT.md: web audit tool design with weighted scoring,
  assembly computed fields, batch AI assistance plan

Makefile

@@ -1,6 +1,7 @@
 .PHONY: build run test clean migrate fmt lint \
         docker-build docker-up docker-down docker-logs docker-ps \
-        docker-clean docker-rebuild
+        docker-clean docker-rebuild \
+        build-calc-oxt install-calc uninstall-calc install-calc-dev test-calc
 
 # =============================================================================
 # Local Development
@@ -19,13 +20,14 @@ run:
 cli:
 	go run ./cmd/silo $(ARGS)
 
-# Run tests
+# Run tests (Go + Python)
 test:
 	go test -v ./...
+	python3 -m unittest pkg/calc/tests/test_basics.py -v
 
 # Clean build artifacts
 clean:
-	rm -f silo silod
+	rm -f silo silod silo-calc.oxt
 	rm -f *.out
 
 # Format code
@@ -158,6 +160,48 @@ uninstall-freecad:
 	rm -f $(FREECAD_MOD_DIR_LEGACY)/Silo
 	@echo "Uninstalled Silo workbench"
 
+# =============================================================================
+# LibreOffice Calc Extension
+# =============================================================================
+
+# Build .oxt extension package
+build-calc-oxt:
+	@echo "Building silo-calc.oxt..."
+	@cd pkg/calc && zip -r ../../silo-calc.oxt . \
+		-x '*.pyc' '*__pycache__/*' 'tests/*' '.gitignore'
+	@echo "Built silo-calc.oxt"
+
+# Install extension system-wide (requires unopkg)
+install-calc: build-calc-oxt
+	unopkg add --shared silo-calc.oxt 2>/dev/null || unopkg add silo-calc.oxt
+	@echo "Installed silo-calc extension. Restart LibreOffice to load."
+
+# Uninstall extension
+uninstall-calc:
+	unopkg remove io.kindredsystems.silo.calc 2>/dev/null || true
+	@echo "Uninstalled silo-calc extension."
+
+# Development install: symlink into user extensions dir
+install-calc-dev:
+	@CALC_EXT_DIR="$${HOME}/.config/libreoffice/4/user/extensions"; \
+	if [ -d "$$CALC_EXT_DIR" ]; then \
+		rm -rf "$$CALC_EXT_DIR/silo-calc"; \
+		ln -sf $(PWD)/pkg/calc "$$CALC_EXT_DIR/silo-calc"; \
+		echo "Symlinked to $$CALC_EXT_DIR/silo-calc"; \
+	else \
+		echo "LibreOffice extensions dir not found at $$CALC_EXT_DIR"; \
+		echo "Try: install-calc (uses unopkg)"; \
+	fi
+	@echo "Restart LibreOffice to load the Silo Calc extension"
+
+# Run Python tests for the Calc extension
+test-calc:
+	python3 -m unittest pkg/calc/tests/test_basics.py -v
+
+# Clean extension package
+clean-calc:
+	rm -f silo-calc.oxt
+
 # =============================================================================
 # API Testing
 # =============================================================================
@@ -219,6 +263,14 @@ help:
 	@echo "  install-freecad-native  - Install for native FreeCAD"
 	@echo "  uninstall-freecad      - Remove workbench symlinks"
 	@echo ""
+	@echo "LibreOffice Calc:"
+	@echo "  build-calc-oxt   - Build .oxt extension package"
+	@echo "  install-calc     - Install extension (uses unopkg)"
+	@echo "  install-calc-dev - Symlink for development"
+	@echo "  uninstall-calc   - Remove extension"
+	@echo "  test-calc        - Run Python tests for extension"
+	@echo "  clean-calc       - Remove .oxt file"
+	@echo ""
 	@echo "API Testing:"
 	@echo "  api-health     - Test health endpoint"
 	@echo "  api-schemas    - List schemas"