Empty cell vs `""` should be equal (matches Excel/Sheets)

[garritfra]

Surfaced while verifying the fix for #68 against Excel/Google Sheets behavior.

Current behavior

When a cell is empty:

• A1=0 → TRUE ✓ (matches Excel/Sheets)
• A1=\"\" → FALSE ✗ (Excel/Sheets return TRUE)

This happens because resolve_cell in crates/cell-sheet-core/src/formula/eval.rs coerces an empty cell-ref to Number(0.0) before it reaches the equality logic:

Expr::CellRef(cell_ref) => {
    let val = resolve_cell(sheet, (cell_ref.row, cell_ref.col));
    if val == CellValue::Empty {
        CellValue::Number(0.0)   // <-- empty becomes 0
    } else {
        val
    }
}

So by the time the typed-equality arm sees the operands, the empty cell is already a number, and Number(0.0) == Text(\"\") is mixed-type → FALSE.

Expected behavior

In Excel / Google Sheets, an empty cell is treated as equal to both 0 (numeric context) and \"\" (text context). That is, empty has a kind of polymorphic identity that the spreadsheet picks based on context.

Possible designs

1. Stop coercing Empty at resolve_cell; let the equality arm handle it. Inside Op::Eq | Op::Neq, treat Empty as equal to Number(0.0), equal to Text(\"\"), and equal to Empty. Other arms (arithmetic, ordering) still coerce empty to 0 explicitly. More invasive but cleaner separation.
2. Special-case Text(\"\") in the equality arm. Currently Number(0.0) = Text(\"\") is FALSE. Add a rule: Number(0.0) = Text(\"\") and vice-versa is TRUE only when one operand originated from an empty cell. Requires tracking origin — ugly.
3. Promote Text(\"\") to Empty when comparing. Localised to the equality arm: if either side is Text(\"\") and the other is Empty or Number(0.0), return TRUE. Simplest.

Option 1 is the most principled but ripples into other places that depend on the current empty-as-zero behavior. Worth a small design discussion before picking.

Acceptance criteria

• A1=\"\" returns TRUE when A1 is empty.
• A1=0 still returns TRUE when A1 is empty (don't regress).
• \"\"=0 — decide and document. (Excel: TRUE. Sheets: TRUE.)
• Tests in cell-sheet-core cover empty/\"\", empty/0, \"\"/0, and ensure non-empty text cells still compare correctly (e.g. A1=\"\" is FALSE when A1 contains "x").
• Help entry for = / <> updated to mention empty/\"\" equivalence.

Out of scope

Whether ISBLANK(A1) should distinguish empty from \"\" — separate question. (In Excel, ISBLANK(\"\") is FALSE but \"\"=A1 is TRUE for empty A1. Asymmetric.)