Engine: a Python cell that keeps only its value keeps its whole cached result

When a file is opened, a Python cell falls back to keeping only its value in
four cases: its code is missing from the Python part or the part is damaged
(the xlsx reader), its code is too long for a cell, its kept result is
malformed, or its result is past the adoption budget. The first kept the
values the cell spilled as plain values; the others dropped them, so values
the file showed were lost. All of them now follow one rule, through
Sheet::settle_spill: the anchor and every spilled cell keep their cached
values as plain values, with no live spill range; a cell's own content and
format are never overwritten; and the work is bounded by the spilled values
stored, not by the range. The values move from the spill into the cells, so
nothing is copied. The cut-by-budget test is renamed for what it now checks,
the dropped-result test now expects the values kept, and the adoption warning
and the file-format docs say so.

docs/file-format-parity.md

@@ -100,7 +100,7 @@ refresh. Counted under features (Get & Transform, ~10% ready), not here.
 | `.xlsx` | read and write | Written as [MS-XLSX] 2.3.10 specifies: the cell's formula is `_xlfn._xlws.PY(i,n)`, where `i` is the index of the cell's script in the workbook's Python part (`xl/python.xml`, `application/vnd.ms-excel.python+xml`) and `n` the return type, 0 for an Excel value or 1 for a Python object, with the cached value. The code keeps its `xl()` references as text, so the formula has no further arguments. A spilling result is written as a dynamic array with its spilled values cached. Read back with or without the `_xlfn.` and `_xlws.` prefixes: an index names a script of the Python part, whose `%Px%` references become the text of the formula's argument `x`, and the older form with the code in string literals, `PY("…"&"…",n)`, still reads. So workbooks laid out as the format specifies open with Python cells, their cached values and spills. Python errors are cached as `#VALUE!`, as Excel caches `#SPILL!` and `#CALC!` |
 | `.xls`, `.xlsb`, `.ods` | read | A `PY("…",0)` or `PY("…",1)` formula read from any format becomes a Python cell; the `.xlsb` and `.ods` imports keep values only, so Python cells there come in as their cached values |
 | GridCraft JSON | read and write | The whole Python cell: code, return type, the last result and the exact error |
-| Limits on open | — | Code that doesn't fit Excel's 8,192-character formula as `PY("…",n)` keeps only its cached value, with a warning, and so does a formula naming a script the Python part doesn't hold. The Python part is read up to 64 MB and 1,000,000 scripts, and the cells take at most 64 MB of code from it in all (several formulas can name one script). A kept result that is not a well-formed array of at most 1,000,000 cells is dropped, a spill range larger than that is ignored, and results are rebuilt for at most 4,000,000 cells over the workbook. Opening runs nothing |
+| Limits on open | — | Code that doesn't fit Excel's 8,192-character formula as `PY("…",n)` keeps only its cached value, with a warning, and so does a formula naming a script the Python part doesn't hold. The Python part is read up to 64 MB and 1,000,000 scripts, and the cells take at most 64 MB of code from it in all (several formulas can name one script). A kept result that is not a well-formed array of at most 1,000,000 cells is dropped, a spill range larger than that is ignored, and results are rebuilt for at most 4,000,000 cells over the workbook. A Python cell that keeps only its value (code too long or not in the part, a damaged part, a dropped result, a result past that budget) keeps the values it spilled as plain values, without overwriting a cell's own content or format. Opening runs nothing |
 | Not kept | — | Python object results stored as rich values (Excel's `richData` parts) come in as their cached values. The Python part's environment definition (its id and initialization code) is not read, and a save writes none |
 
 ## Work (76–114 h)