Tracer bullet: add-in skeleton + popup + Name Manager injection

[jimmytacks]

Part of #1

Description

Build the end-to-end tracer bullet: press Ctrl+Shift+L → popup appears → press Enter → a hardcoded LAMBDA appears in the workbook's Name Manager.

Deliverables

• AddIn.cs — ExcelDNA lifecycle: AutoOpen, keyboard shortcut, deferred init via QueueAsMacro
• LambdaLoader.cs — InjectLambda(name, formula) via Workbook.Names.Add COM interop
• LambdaPopup.xaml/.cs — Minimal WPF window with hardcoded list, keyboard navigation (Escape/Up/Down/Enter)
• Project files — .csproj, .dna, nuget.config, CI workflow
• Test library — lambdas/test/ with Double.lambda, Triple.lambda, AddN.lambda
• Test projects — Unit tests + AddIn smoke tests (load add-in, verify shortcut, verify Name Manager injection)

Discovery questions to answer

• Does Names.Add require a macro context (QueueAsMacro)?
• Does the formula string need the = prefix?
• How to handle existing names (update vs error)?

Depends on

• Extract shared ExcelDNA infrastructure from Formula Boss taglo-excel-common#1 (shared package published)

See also: #1