Refactor to LET — PR 1: tracer (non-LET formulas, refs + ranges + spill)

[jimmytacks]

Part of #201

Tracer-bullet slice. End-to-end loop on the simplest case: a non-LET formula with cell refs and ranges, including spill refs (A1#) distinct from their anchor (A1). Existing LETs are refused with a "coming in PR 2" message — removed in #PR2.

Scope

• Shared-infra change: add bool IsSpilled to FormulaRef (equality + hash + DisplayAddress); CellRefExtractor.BuildFormulaRef populates it from the existing (?<spill>\#) regex group. Verify existing CellRefExtractorTests and GatherEngineTests still pass. If any /Gather test fails due to double-counting A1 vs A1#, fall back to a RefactorEngine-local key wrapper instead.
• Register /Refactor slash command in LambdaPopup.xaml.cs. On an empty / literal active cell, popup closes silently (matches /Gather's pattern).
• New RefactorEngine.Refactor(string formula, string activeSheet) walks via CellRefExtractor.Extract, dedupes by FormulaRef, assigns inputN auto-names, rewrites via CellRefExtractor.Rewrite, emits LET via FormulaFormatter.AppendLet.
• New RefactorToLetWindow: original-formula header (read-only), inputs section with rename + Include checkbox + reorder (Alt+Up/Down + drag, reusing KeptRowReorderHandler), preview pane, Save/Cancel. No promotable section, no calc-binding section in this PR.
• New Commands/RefactorCommand.cs — mirrors GatherCommand for slash-command wiring and active-cell write-back via Formula2.
• Existing LET refused via RefactorDiagnostic of kind ExistingLet. Slash command shows the diagnostic via MessageBox.
• External refs handled in-line as inputs in this PR (same as /Gather today). PR 3 moves them into the promotable section.

Files

New:

• addin/lambda-boss/RefactorEngine.cs
• addin/lambda-boss/RefactorTypes.cs
• addin/lambda-boss/UI/RefactorToLetWindow.xaml
• addin/lambda-boss/UI/RefactorToLetWindow.xaml.cs
• addin/lambda-boss/Commands/RefactorCommand.cs
• addin/lambda-boss.Tests/RefactorEngineTests.cs
• addin/lambda-boss.Tests/CellRefExtractorSpillTests.cs

Modified:

• addin/lambda-boss/GatherTypes.cs (add IsSpilled to FormulaRef)
• addin/lambda-boss/CellRefExtractor.cs (populate IsSpilled)
• addin/lambda-boss/UI/LambdaPopup.xaml.cs (register /Refactor)

Tests (RefactorEngineTests)

• single cell ref
• multiple refs deduped
• range
• spill-distinct (A1 and A1# produce two bindings)
• sheet-qualified ref dedupe (A1 and Sheet1!A1 collapse when active sheet is Sheet1)
• cross-sheet ref stays distinct
• rename through body
• round-trip via LetParser.Parse + LetToLambdaBuilder.Build

Manual Excel test

Type =IF(A1<10, IF(A1>2, SUM(B1:B5), 0), SUM(B2:B6)) into a cell. Run /Refactor. Save. Confirm the resulting LET evaluates identically.

Reference

Plan: plans/0008-refactor-to-let.md § PR 1