Smooth tool: smoothing over a closed path's first anchor keeps its start

When a drag reached the first anchor of a closed path, the smoothed path
started at the first anchor past the drag, and a drag over the whole
path could start it at a corner the refit kept. Dashes and type on the
path then started somewhere else.

The smoothed path now starts at its point nearest where the first anchor
was: at an anchor there, or at a new anchor split into the segment
there, which leaves the outline as it is. Of points equally near, the
one earliest along the path before smoothing, from its first anchor,
is used. A drag that does not reach the first anchor leaves it first,
as before.

Part of #2224.

docs/target-app-parity.md

@@ -1118,6 +1118,7 @@ Moved from the ROADMAP's "Shipped so far" on 2026-10-10. Update it when a featur
     The Pencil's and the Paintbrush's Fidelity is in screen pixels, the same at any zoom: their Tool Options give it in px, from 0.01 px on the logarithmic slider to 20 px and up to 100 px in the field, and the tool sends `path.freehand` the value in points at the current zoom (#5729). The Smooth tool's and the Blob Brush's Fidelity stays in points.
     `path.freehand` and `brush.freehand` take a Fidelity of up to 100 / 0.0313 pt (about 3194.89 pt), the 100 px field maximum at the lowest zoom (3.13 %), and reject a value outside their range with an error, so a Pencil or Paintbrush stroke drawn zoomed out keeps the Fidelity set in pixels (engine tests `pencil_fit_keeps_its_anchors_on_a_stroke_drawn_zoomed_out` and `pencil_takes_fidelity_within_its_range_and_rejects_the_rest`).
     Each drag of the Smooth tool smooths the stretch of a selected path in its reach along its length before refitting it within Fidelity, so a wobble wider than Fidelity is smoothed out and another drag over the same place smooths it further, as [the help page](https://helpx.adobe.com/illustrator/using/tool-techniques/smooth-tool.html) describes (`path.smoothRegion`). The smoothing spread (a third of the reach) and the rule that each refit stays within twice the distance the smoothing moved the path, so repeated drags do not drift from it, are VectorCraft's own. A drag over the whole of a closed path smooths it across its first anchor too, so a bump there is smoothed like any other and no corner is left; Object › Path › Smooth smooths a closed path across its first anchor the same way.
+    When a Smooth tool drag reaches the first anchor of a closed path, the smoothed path starts at its point nearest where that anchor was: at an anchor there, or at a new anchor split into the curve without changing its outline. Dashes and type on the path start from that point. A drag that does not reach the first anchor leaves it first. This is VectorCraft's own rule (engine tests `smoothing_over_a_closed_paths_first_anchor_keeps_its_start` and `smoothing_a_whole_closed_path_with_a_spike_keeps_its_start`).
     Before a press that draws a new path, the Pencil cursor shows a small star, as [Draw freeform paths with the Pencil tool](https://web.archive.org/web/20260926095638/https://helpx.adobe.com/illustrator/desktop/draw-shapes-and-paths/draw-shapes/draw-freeform-paths-with-the-pencil-tool.html) describes; over a selected path within Edit selected paths' distance, which the press continues from an end or redraws, the star is not shown ([Reshape paths with the Pencil tool](https://web.archive.org/web/20260106004407/https://helpx.adobe.com/illustrator/desktop/draw-shapes-and-paths/draw-shapes/reshape-paths-with-the-pencil-tool.html)).
     Keep selected in the Pencil and Paintbrush Tool Options is on by default: while it is on, a stroke selects the path it draws or continues and replaces the earlier selection. While it is off, a new stroke leaves nothing selected, the earlier selection included, as the reference application starts the stroke by dropping it (#5752); a continued or redrawn path is deselected while the rest of the selection stays (`path.freehand` `keepSelected`).
     With two open paths selected, a Pencil stroke from an end of one that ends within Edit selected paths' distance of an end of the other connects them into one path with the first path's appearance, in one undo step, as [Connect two paths with the Pencil tool](https://helpx.adobe.com/illustrator/desktop/draw-shapes-and-paths/draw-shapes/connect-two-paths-with-the-pencil-tool.html) describes (`path.freehand` `connect`).