Update capability: re-fetch and overwrite loaded LAMBDAs

[jimmytacks]

Part of #1

Description

Re-fetch LAMBDAs from GitHub and overwrite already-loaded Name Manager definitions.

Deliverables

• WorkbookTracker.cs — Track which LAMBDAs were loaded into which workbook, from which repo/library, with which prefix. Session-only (not persisted)
• LambdaLoader.cs — Add UpdateLibrary method: re-fetch from GitHub, overwrite Name Manager definitions, return diff (new/updated/unchanged)
• LambdaPopup.xaml — Add "Update" action for loaded libraries, show change summary after update

Tests

• AddIn test: load library, modify a .lambda file (or mock), trigger update, verify Name Manager reflects changes

Depends on

• Settings persistence and multi-repo management #6 (settings and multi-repo)