Path ops: stroke outlining and unions sweep long, low paths along their length

The overlap sweep moves a horizontal line down the page, and its time
per event grows with the number of segments on that line. A long, low
path, such as rows of a zigzag, puts a whole row on the line at once.

stroke_region, unite_all_with and try_unite_all_with now estimate how
many segments the sweep holds at once in each direction, from up to
4,096 segment boxes (one from each run of segments, at a position that
changes from run to run). When the estimate for the input turned a
quarter turn is under two thirds of the upright one (2,048 segments or
more), the operation runs on the turned input and the result is turned
back. A quarter turn swaps and negates coordinates, so no point moves
by rounding. Offset Path is not turned. Callers of unite_all,
try_unite_all and outline_stroke take the turned sweep through these
functions.

Tidy's lookups of the input curves around a piece and of the kept
anchors can search along y: once lookups with more than 64 candidates
along x have scanned 16 per item, a lookup sorts the list along y once
and uses that window when it holds fewer candidates (under a quarter as
many for curves). Where the y window rarely helps, only one lookup in
32 measures it. is_kept counts the few points within its tolerance
directly instead of searching for the window's end.

In the sweep_axis tests, a turned result has as many subpaths as the
result without the turn, and the area where the two differ is no
larger than the refit precision times the outline's length.

On one open path of 200,000 anchors in rows of 1,000, Outline Stroke
takes 17.3 s of processor time instead of 31.3 s and Unite 2.9 s
instead of 6.5 s in a release build. Unite of 2,000 circles takes the
same time as before.

docs/target-app-parity.md

@@ -960,6 +960,7 @@ Moved from the ROADMAP's "Shipped so far" on 2026-10-10. Update it when a featur
 - **Geometry and effects:**
   - **Open-path anchor Delete:** Delete/Backspace removes selected anchors and their adjoining segments, leaving drawable runs as separate open subpaths of the original object. Surviving curves and appearance stay intact; one Undo restores the document and selection, and a second Delete removes the selected remainder. Explicit Remove Anchor Points still reconnects/refits. Four core regressions, one full-frame Delete/Backspace regression and three existing removal/segment controls pass, covering these distinctions, multiple deletions, transformed geometry and mixed whole-object selection. Behaviour references: the legacy public [Illustrator reference guide, Add or delete an anchor point](https://helpx.adobe.com/content/dam/help/en/pdf/illustrator_reference.pdf) distinguishes keyboard deletion from dedicated removal; the current [Add or remove anchor points](https://helpx.adobe.com/illustrator/desktop/draw-shapes-and-paths/modify-paths/add-or-remove-anchor-points.html) describes the dedicated tools. Other G11 gaps and scores/hours remain unchanged.
   - Pathfinder (10 exact curve booleans; single-result shape operations retain source names and SVG IDs, #1293), Offset, Outline Stroke, Simplify, Clean Up, Split Into Grid, Divide Objects Below. With one group selected alone, the Pathfinder panel's buttons work on its members (among other selected objects a group still acts as one shape), and a greyed-out button says what it needs (#1450; behaviour reference: [combine objects using Pathfinder effects](https://helpx.adobe.com/illustrator/desktop/manage-objects/reshape-transform-objects/combine-objects-using-pathfinder-effects.html); one plain path selected alone stays disabled, which the documentation doesn't settle). Split Into Grid takes the number, height or width, gutter and total of the rows and of the columns, Add Guides and Preview, and several selected paths become one grid painted as the topmost one (#1462, `object.path.splitIntoGrid`; behaviour reference: [divide or split objects](https://helpx.adobe.com/illustrator/desktop/manage-objects/edit-objects/divide-or-split-objects.html); a typed height or width keeps the gutter and changes the total, our choice). Object › Path › Remove Anchor Points (`path.removeAnchors`) removes the direct-selected anchors without opening their paths (the Delete key still deletes them with their segments): the neighbours keep their handle directions and their facing handles are refitted so one cubic follows the two old segments, and straight sides stay straight. The Delete Anchor Point tool uses the same refit. The Control bar, the contextual task bar, the Properties quick actions and the canvas context menu offer it when anchors are selected. With anchors selected, the Control bar and the Properties panel also convert them to corner or smooth (`path.convertAnchors`), connect end points (Join) and cut the path at them (`path.cutAtAnchors`, also on the task bar and the context menu): a closed path opens there and an open one becomes one path per piece, each cut leaving one of its two coincident anchors selected so a drag pulls the path apart. In the Pathfinder panel a group selected alone counts its nested shapes, so an expanded stroked object keeps the Shape Modes enabled (#1642, #1635).
+    Outline Stroke and Unite remove the overlaps of a long, low path with the sweep turned along it: when the estimated number of segments the sweep holds at once is under two thirds as large for the path turned a quarter turn (paths of 2,048 segments or more), the overlaps are removed on the turned path and the result is turned back, which moves no point by rounding. The lookups of the input curve that a piece of the result lies on, and of the input anchors that are kept, can search along y instead of x: once lookups with more than 64 candidates along x have counted 16 for each input curve or anchor, a lookup searches along y when it finds fewer candidates there (under a quarter as many for curves), and where that has rarely happened it measures along y in only one lookup of 32. On one open path of 200,000 anchors in rows of 1,000, Outline Stroke takes 17.3 s of processor time instead of 31.3 s and Unite 2.9 s instead of 6.5 s in a release build. A turned result has as many subpaths as the result without the turn, and the area where the two differ is no larger than the refit precision times the outline's length (`crates/pathops/src/sweep_axis.rs` tests).
   - Divide uses open lines, bent paths and curves as cutting edges (#1307), preserving the frontmost filled face's appearance and holes. Engine tests cover implicit closure of filled open contours and unrelated objects; native automation confirms independent piece movement, one Undo/Redo, canvas/window captures and an exact saved-document read by a separate CLI process. The shared planar map keeps a segment cap (65,536 since #937, was 4,096) and its filled-area fallback; other Pathfinder operations are unchanged.
   - Type that Transform effects only move and copy (no scale, rotation, reflection or Random) exports as type in each place, and PDF export writes every copy as selectable, searchable text, when its characters are in solid colors without underline or strikethrough, the type object has no fills or strokes of its own, no inline graphics and no perspective, its transform does not scale or shear stroked characters, and the copies do not overlap. When the effect is on the type itself, the type can also have transparency, an opacity mask and raster effects. When the effect is on the group or layer holding the type, the type has no transparency or effects of its own and is not in a clipping path, the group or layer has no fill, stroke or Pathfinder effect of its own, and no group between them has a fill, stroke, Pathfinder or geometry effect. Type still exports as outlines when the effect stops its copies early, or when the copies would number more than 10,000 or take more than 20,000,000 bytes of document data, and so does other type under a geometry effect.
   - Extrude & Bevel and Inflate treat strokes as outlined: each visible stroke builds a solid from its outline in its own paint beside the top fill's ([three-d.md](three-d.md#fills-and-strokes)).