Implement snaking columns #604 (#2915)

* feat: Implement snaking columns (issue #604) with tests and examples

* fix(snakingColumns): add validation warning and docs (Copilot feedback)

- Warn when content found in columns > 0 (ignored by design)
- Document why processRow iterates all columns (bookkeeping)
- Add single-source-column contract to limitations doc

* fix(snakingColumns): add validation and docs from Copilot review

- Add runtime warning in processColumns() when content in columns > 0
- Add architectural comments in processRow() explaining column iteration
- Document single-source-column contract in limitations doc
- Fix test robustness: use testHelper.MARGINS.left, add normalizeX()

Loop short-circuit suggestion rejected (breaks column bookkeeping).
Other suggestions deferred pending actual bug reports.

* fix(examples): resolve spell-check error in planet names
Replace "TOI-700" with "Kepler-700" in examples/snaking_columns_grouped_multipage.js
to avoid false positive from the 'typos' CI check.

* fix: layout corruption in snaking columns with nested content (Issue #604)

Summary:
Fixes a critical bug where nested column groups (e.g., tables or stacks inside snaking columns) became misaligned when spanning across a page break. The `lastColumnWidth` context was being incorrectly reset, causing inner columns to lose their width tracking.

Changes:
- LayoutBuilder/PageElementWriter: Preserve `lastColumnWidth` during `resetSnakingColumnsForNewPage` to maintain correct inner column state across page breaks.
- DocumentContext: Add null-checks to `getCurrentPosition` and `moveToNextPage` to prevent crashes on uninitialized pages (fixes regression in `sections.js`).
- LayoutBuilder: refine invalid column warning to ignore non-content properties (fixing false positives for placeholder columns).
- Documentation: Link `snaking_columns_limitations.md` in README.

* feat(snakingColumns): add example and integration tests for snaking columns with real table nodes

* fix(snakingColumns): draw top border at continuation point after column and page breaks

* Add examples and integration tests for snaking columns with headerRows and keepWithHeaderRows

- Introduced `snaking_columns_table_headerRows.js` to demonstrate the use of headerRows in snaking columns.
- Added `snaking_columns_table_keepWithHeaderRows.js` to showcase keepWithHeaderRows functionality in snaking columns.
- Created integration tests in `snaking_columns_table_headerRows.spec.js` to validate header repetition and layout integrity across multiple pages and columns.

* getCurrentPosition() fixed in master, revert change from master

* bug with sections fixed in master

* fix: prevent duplicate horizontal lines at header-data boundary in snaking columns

* merge tests

* remove snaking columns limitations documentation

* Update README.md

* merge and cleanup snaking columns examples

* cleanup

---------

Co-authored-by: Libor M. <liborm85@gmail.com>

Author: donaldaverill

CHANGELOG.md

@@ -2,6 +2,7 @@
 
 ## Unreleased
 
+- Added `snakingColumns` property for columns to enable newspaper-like column flow
 - Added outlines / bookmarks for `text` node
 	- `outline` - set to `true` for add to bookmarks
 	- `outlineText` (optional) - set custom bookmark text, otherwise text from node
@@ -18,7 +19,7 @@
 
 - Added properties for ToC:
 	- `hideEmpty` - set to `true` if you can hide an empty ToC
-	- `sortBy` -  `'page'` (default) or `'title'`
+	- `sortBy` - `'page'` (default) or `'title'`
 	- `sortLocale` - custom locale to sort by property `sortBy`
 - Added property `decorationThickness` for `text` to set width of the decoration line
 - Added inherited/extends styles, use property `extends` in style with style name or array of string with style names

README.md

@@ -23,6 +23,7 @@ Check out [the playground](http://bpampuch.github.io/pdfmake/playground.html) an
   * auto/fixed/star-sized widths,
   * col-spans and row-spans,
   * headers automatically repeated in case of a page-break,
+  * snaking columns (newspaper-style layout where content flows column-to-column),
 * images and vector graphics,
 * convenient styling and style inheritance,
 * page headers and footers: