Unnest to LET (spec 0009)

[jimmytacks]

Parent issue for spec 0009 — Unnest to LET (specs/0009-unnest-to-let.md).

A new /Unnest slash command that takes a nested formula and explodes each function call and operator expression into a named LET step (leaf-first), making intermediate values inspectable for debugging. It is the complement to /Refactor (spec 0008): /Refactor hoists leaves into input bindings; /Unnest hoists intermediate calculations into step bindings. The two compose — /Unnest then /Refactor then /LET to LAMBDA.

Key design decisions (approved):

• Separate, composing command — reuses RefactorEngine's existing calc-binding ref-hoisting for the input half (zero new code there).
• Max granularity, then collapse — every function/operator node becomes a step; an Include toggle inlines unwanted ones.
• Function-derived step names — sumsq1, sqrt1, round1; operator steps calcN.
• No CSE in v1 — the step tree mirrors the syntax tree.
• Introduces the repo's first recursive-descent formula parser (AST) — the main new engine and risk area.

Tasks

• Unnest: formula expression parser (AST) #270 Formula expression parser (AST) — foundation, no UI, heavy edge-case tests
• Unnest: decomposition engine (non-LET) #271 Decomposition engine (non-LET) — AST -> leaf-first LET steps
• Unnest: UnnestToLetWindow dialog + /Unnest command #272 UnnestToLetWindow dialog + /Unnest command — UI, wiring, AddinTests
• Unnest: existing-LET handling #273 Existing-LET handling — explode body + calc-binding RHSs

Build order: #270 -> #271 -> #272 -> #273.