Formulas: flat operator chains as long as Excel allows (#916)

along: =1+1+... with 4,096 operands (8,192 characters, exactly Excel's
limit, which Excel accepts and calculates) overflowed the stack when
entered, because the derived Clone of Expr recursed once per operand
down the chain's left spine. Saving hit the same in the XLSX writer.

- Expr: Clone, PartialEq, walk and map follow the left spine in a loop
  and recurse only into right operands and real nesting.
- XLSX: _xlpm. prefixing and the implicit-intersection scan follow the
  spine too. The scan's depth limit of 64 wrote any chain of more than
  about 64 operands as a dynamic-array formula; such sums are plain
  formulas again (ranges in the chain still get array evaluation).
- Evaluate Formula borrows sub-expressions instead of cloning each
  operator's subtree (quadratic memory on a long chain).
- Spanish formula entry and display no longer refuse chains of more
  than 256 operands; the left spine does not count as nesting.

Tests run on a 1 MiB stack (the Windows main thread) with Excel's
observed results: formula (clone/eq/walk/map/adjust at 4,096), calc
(401 and 4,096 operands, -, *, /, ^, &, %, comparisons, 1,500 refs),
xlsx (long chains written as scalar formulas), engine (cell.set, fill,
copy/paste, insert rows/columns, undo/redo, Evaluate Formula, XLSX save
and reopen), Spanish entry. Each fails without the fix.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/function-parity.md

@@ -1,6 +1,6 @@
 # Function parity: worksheet functions and formulas
 
-> **Last reviewed:** 2026-10-15 · **Last updated:** 2026-10-15 · **Change:** minor (MEDIAN and AGGREGATE median avoid intermediate overflow) · **Target:** Microsoft Excel (Microsoft 365), Excel for Mac 16.113.4
+> **Last reviewed:** 2026-10-15 · **Last updated:** 2026-10-15 · **Change:** minor (flat operator chains as long as Excel allows: 4,096 operands no longer overflow the stack, Spanish entry accepts them, files keep them as plain formulas); previously minor (MEDIAN and AGGREGATE median avoid intermediate overflow) · **Target:** Microsoft Excel (Microsoft 365), Excel for Mac 16.113.4
 
 Excel's worksheet functions, category by category, against GridCraft's function library. Part of
 [target-app-parity.md](target-app-parity.md); gaps feed [gaps.md](gaps.md).
@@ -84,6 +84,7 @@ PIVOTBY, REGEXTEST…) are all present.
 | 15-significant-digit comparison semantics | **missing** (#57: `=0.1+0.2=0.3` is FALSE) |
 | External workbook references (`[Book.xlsx]Sheet1!A1`) | parsed; links dropped on open, no update |
 | Localized function names and `;` separators | **missing** (#45); en-US only |
+| Long flat operator chains (`=A1+A2+…`, `-`, `*`, `/`, `^`, `&`, comparisons) | done: no operand limit below Excel's 8,192-character formula limit (Excel, observed: `=1+1+…` with 4,096 operands is exactly 8,192 characters with the `=` and calculates; one more character is refused). Evaluated (#484), printed (#628), stored, copied, adjusted, saved and reopened without recursing once per operand, on a 1 MiB stack (#916); written to XLSX as plain formulas, not dynamic arrays; Spanish entry accepts them |
 | Error values | the seven classic errors plus `#SPILL!`, `#CALC!`, `#GETTING_DATA` done; `#FIELD!`, `#BLOCKED!`, `#CONNECT!`, `#BUSY!`, `#UNKNOWN!`, `#PYTHON!` missing (minor: they come from data types, cloud and Python features) |
 
 ## Depth (estimated)
@@ -114,6 +115,7 @@ workbooks; results checked in as data, not Excel files) and fixing what it finds
 
 | Date | Change | Summary |
 |---|---|---|
+| 2026-10-14 | minor | Long flat operator chains (#916): 4,096 operands (Excel's 8,192-character limit) no longer overflow the stack when entered, filled, adjusted or saved; chains past 64 operands are no longer saved as dynamic-array formulas; Spanish entry no longer refuses chains past 256 operands |
 | 2026-10-15 | minor | MEDIAN and AGGREGATE median use an overflow-safe midpoint (#1579); regressions cover finite extremes, tiny values and dependent recalculation |
 | 2026-10-13 | minor | Clear reports changed stored cells sparsely, including those beyond the first million coordinates; dependent formulas and removed spills update, while clearing empty merges still unblocks spills (#1032) |
 | 2026-10-12 | minor | USDOLLAR added as a Compatibility function (not on Microsoft's list, so the counts don't change); MIRR with no positive flow is -1, COUNTIF's bare `"<="` / `">="` skip empty strings, INDIRECT with `a1` = FALSE gives `#REF!` for A1-style text (#738) |