Docs: Python cells as data (architecture, file formats, functions, roadmap)

architecture.md gains a Python cells section: the model type, the text
forms and the size limit, the xlsx Python part, calc laying out kept
results as graph nodes, and the limits on open. file-format-parity.md
gains a Python cells table (xlsx as [MS-XLSX] 2.3.10 specifies, the
formula naming its code in the Python part, with cached values and
spills; other formats; GridCraft JSON; the limits on open; and
rich-value object results as not kept). function-parity.md marks the
four Python errors done with their ERROR.TYPE numbers and adds a Python
in cells row (stored, not run). ROADMAP.md records the progress. The
return types are written as "0 or 1" so no table cell holds an
unescaped pipe. architecture.md's "Last updated" label loses the space
inside its bold markers.

docs/function-parity.md

@@ -1,6 +1,6 @@
 # Function parity: worksheet functions and formulas
 
-> **Last reviewed:** 2026-10-15 · **Last updated:** 2026-10-25 · **Change:** minor (the LAMBDA row lists the recursion limit and its `#NUM!`, and the error a defined name holding an error gives when called; earlier: number text with accounting parentheses coerces like Excel 365; INDEX with one index on a 2-D reference is #REF! as in Excel; earlier: SUBTOTAL and AGGREGATE cap the cells they visit, #2175) · **Target:** Microsoft Excel (Microsoft 365), Excel for Mac 16.113.4
+> **Last reviewed:** 2026-10-15 · **Last updated:** 2026-10-25 · **Change:** minor (the Python errors `#PYTHON!`, `#BUSY!`, `#TIMEOUT!` and `#BLOCKED!`, and a Python in cells row; earlier: the LAMBDA row lists the recursion limit and its `#NUM!`, and the error a defined name holding an error gives when called; earlier: number text with accounting parentheses coerces like Excel 365; INDEX with one index on a 2-D reference is #REF! as in Excel; earlier: SUBTOTAL and AGGREGATE cap the cells they visit, #2175) · **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).
@@ -93,7 +93,8 @@ This improves series preparation; it does not change the model-fitting or season
 | 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) |
+| Error values | the seven classic errors plus `#SPILL!`, `#CALC!`, `#GETTING_DATA`, and the Python errors `#PYTHON!`, `#BUSY!`, `#TIMEOUT!`, `#BLOCKED!` done; `#FIELD!`, `#CONNECT!`, `#UNKNOWN!` missing (minor: they come from data types and cloud features). `ERROR.TYPE` gives 11 for `#BLOCKED!` as Excel does; Microsoft documents no number for `#PYTHON!`, `#BUSY!` or `#TIMEOUT!`, and GridCraft gives them 19, 20 and 21 |
+| Python in cells | partial: Python cells are stored, edited (`python.set`), copied, moved and saved, and Excel's `_xlfn._xlws.PY` cells open with their cached results; the code doesn't run, so new or changed code shows `#BLOCKED!`. Typing `=PY(` is refused with a pointer to `python.set`, so `PY` stays counted as missing above |
 
 ## Depth (estimated)
 
@@ -123,6 +124,7 @@ workbooks; results checked in as data, not Excel files) and fixing what it finds
 
 | Date | Change | Summary |
 |---|---|---|
+| 2026-10-25 | minor | Python in cells (stored, not run): `#PYTHON!`, `#BUSY!`, `#TIMEOUT!` and `#BLOCKED!` done, with their `ERROR.TYPE` numbers; `PY` still counted as missing |
 | 2026-10-25 | minor | LAMBDA row: a LAMBDA that calls itself more than 140 times, and a formula nested past the depth limit, give `#NUM!` (#1809; nesting gave `#CALC!` before). `=Foo(1)` where the defined name Foo holds `#N/A` gives `#N/A`, not `#NAME?` |
 | 2026-10-23 | minor | Number text coercion (VALUE, arithmetic, typed entry): a dollar sign before accounting parentheses (`$(1,234)`, `$ (5)`) and a percent sign after them (`(5)%`) read as negatives; `(-5)`, `((5))`, `($-5)`, `$5%`, `$(5)%` and dates or times in parentheses are `#VALUE!`, as observed in Excel 365 |
 | 2026-10-23 | minor | INDEX with only a row index on a reference of several rows and columns (`INDEX(A1:C3,2)`, `INDEX(A1:C3,0)`) is `#REF!`, as observed in Excel, so it also fails as a range endpoint and inside other functions; an empty column argument (`INDEX(A1:C3,2,)`) still gives the row. Checked that a column array with one index stays a 1×1 array (`TYPE` 64), which is what Excel returns |