Unnest: decomposition engine (non-LET)

[jimmytacks]

Part of #269 — spec 0009 (specs/0009-unnest-to-let.md). Depends on #270 (parser).

Pure logic + unit tests: turn an AST into a leaf-first =LET(...) of named steps. No UI.

Scope:

• Step extraction — every function-call node and every binary-operator-expression node (other than the root) becomes a step. Unary expressions and postfix %/# stay inline. Atomic leaves stay inline (they are /Refactor's job).
• Root as body — the outermost node becomes the LET body (not a named step), with its child references rewritten to step names.
• Leaf-first ordering — each step precedes every use of it (topological).
• No CSE in v1 — repeated identical sub-expressions each get their own step.
• Naming — function-call steps auto-name from the lowercased function name (sumsq1, sqrt1, round1); operator steps calcN. Smallest numeric suffix that avoids collision with other steps, existing binding names, and workbook defined names; a base used once still gets 1.
• Include-toggle inlining — un-including a step substitutes its RHS back into the parent and re-parents its children to the grandparent.
• Emit via FormulaFormatter.AppendLet; result must round-trip through LetParser.

Worked example (from spec)

=ROUND(SQRT(SUMSQ(XLOOKUP(H94, t[City], t[[X-Coordinates]:[Y-Coordinates]]) - $I$92:$J$92)) * 100, 0)
->

=LET(
    xlookup1, XLOOKUP(H94, t[City], t[[X-Coordinates]:[Y-Coordinates]]),
    calc1,    xlookup1 - $I$92:$J$92,
    sumsq1,   SUMSQ(calc1),
    sqrt1,    SQRT(sumsq1),
    calc2,    sqrt1 * 100,
    ROUND(calc2, 0))

Tests

• Worked example
• Function-only formula; operator-only formula
• Single-leaf / single-root no-op (e.g. =A1+B1 -> body only, zero steps)
• Function-derived + calcN naming with collision suffixing
• Include-toggle inlining + child re-parenting
• Round-trip through LetParser