Sketcher: regular polygon can collapse into a degenerate shape when circle centers are constrained coincident to polygon vertices

### Problem description

**Problem description**

While building a circular hole pattern in Sketcher, a regular construction polygon can occasionally collapse into a different degenerate shape when a circle center is constrained coincident to one of the polygon vertices.

Workflow:

1. Create a regular polygon in Sketcher and mark it as construction geometry.
2. Use the polygon vertices as a placement guide for a circular hole pattern.
3. Create a hole circle away from the polygon.
4. Select the hole circle center and a polygon vertex.
5. Apply a coincident constraint.
6. Repeat for additional holes.

Expected result:

- The selected circle center moves onto the selected polygon vertex.
- The regular polygon remains a 10-sided polygon.

Actual result:

- Sometimes the polygon suddenly collapses into a different solved state.
- In my case, the original 10-gon became a degenerate shape with repeated vertices / duplicated edges and looked like it had dropped to 8 points.
- `Ctrl+Z` restores the sketch.
- Retrying the exact same action sometimes works and sometimes reproduces the collapse again.

This is disruptive because the workflow appears normal until the sketch solver jumps to a different valid-but-unintended configuration.

I was able to capture a failure case with a macro and a crashed project file.

Recorded macro excerpt from the failure:

```python
App.getDocument('Unnamed').getObject('Sketch003').addConstraint(Sketcher.Constraint('Coincident',28,3,8,2))
```

That failing step was recorded after several successful circle-to-vertex coincidences in the same sketch.

Observed project state:

- Original polygon sketch geometry was 10 line segments plus a construction circle.
- After the failure, the polygon line segments were still present, but the solved geometry had collapsed to 8 unique vertex locations, with repeated/duplicated edges.
- The geometry was not deleted; it was solved into a degenerate alternate configuration.

This suggests the regular polygon construction may be underconstrained or ambiguously constrained in a way that allows alternate equal-chord solutions on the same construction circle, and the added coincident constraints can push the solver into one of those unintended solutions.

**Reproduction recipe**

1. Open the attached project file before the failure if available, or use the attached failed project to inspect the resulting state.
2. Edit `Body001.Sketch003` (`Roller Sketch`).
3. Create a detached circle.
4. Select the circle center and then a regular polygon vertex.
5. Apply `Coincident`.
6. Repeat until the polygon collapses.

In the captured failure sequence, the macro recorded these successful/failing coincidence additions:

```python
App.getDocument('Unnamed').getObject('Sketch003').addConstraint(Sketcher.Constraint('Coincident',24,3,3,2))
App.getDocument('Unnamed').getObject('Sketch003').addConstraint(Sketcher.Constraint('Coincident',25,3,4,2))
App.getDocument('Unnamed').getObject('Sketch003').addConstraint(Sketcher.Constraint('Coincident',0,1,26,3))
App.getDocument('Unnamed').getObject('Sketch003').addConstraint(Sketcher.Constraint('Coincident',5,2,27,3))
App.getDocument('Unnamed').getObject('Sketch003').addConstraint(Sketcher.Constraint('Coincident',28,3,8,2))
```

The last one is the step after which the polygon collapsed in the saved failure case.

**Full version info**

Please replace this section with the exact output from `Help > About FreeCAD > Copy to clipboard`.

Example context for this report:

```text
FreeCAD 1.1.1
Sketcher workbench
```

**Subproject(s) affected**

- Sketcher

**Anything else?**

Artifacts available for attachment:

- Macro:
`/home/me/.local/share/FreeCAD/v1-1/Macro/Add circle to polygon until crash.FCMacro`
- Failed project:
`/home/me/Documents/PPump -crashed.FCStd`

Local analysis of the failed file:

- The failed project contains `Sketch003` labeled `Roller Sketch`.
- The regular polygon constraints are still present after failure:
- closed chain coincidences
- equal-length constraints
- point-on-construction-circle constraints
- The polygon geometry was solved into a degenerate state rather than deleted.
- The failure did not look like a simple wrong-selection bug; the recorded low-level coincident constraint targeted the expected circle center and polygon vertex.

If useful, I can also provide a minimal analysis summary of the sketch state before/after failure.

### Workbench affected?

Sketcher

### Steps to reproduce

[Case files.tar.gz](https://github.com/user-attachments/files/30056172/Case.files.tar.gz)

### Expected behavior

The circle should be added to the polygon vertex selected.

### Actual behavior

The polygon changes from a 10 point polygon to an 8 point polygon.

Before:

After:

### Development version About Info (in Safe Mode)

```shell
OS: Fedora Linux 44 (Workstation Edition) (KDE/plasma/wayland)
Architecture: x86_64
Version: 1.1.1.Unknown
Build date: Unknown
Build type: Unknown
Python 3.14.5, Qt 6.11.1, Coin 4.0.7, Vtk 9.5.2, boost 1_90, Eigen3 5.0.1, PySide 6.11.1
shiboken 6.11.1, SMESH 7.7.1.0, xerces-c 3.3.0, OCC 7.9.1
Locale: English/United States (en_US)
Navigation Style/Orbit Style/Rotation Mode: CAD/Trackball/Drag at cursor
Stylesheet/Theme/QtStyle: unset/FreeCAD Classic/
Logical DPI/Physical DPI/Pixel Ratio: 96/91.9477/1
Installed mods:
* 3D_Printing_Tools
* QuickMeasure 2022.10.28
* A2plus 0.4.68
* CurvedShapes 1.0.14
* MakerWorkbench 1.0.1
* Curves 0.6.71
* Assembly4 0.61.0
* CfdOF 1.36.3
* freecad.optics_design_workbench 0.7.3
* Plot 2026.4.15

This build is also available here:
https://copr.fedorainfracloud.org/coprs/linuxguy123/freecad
```

### Last known good version (optional)

```shell

```

Start by reproducing the failure in FreeCAD Sketcher with Body001.Sketch003 (Roller Sketch), using the attached Case.files.tar.gz and the recorded macro constraints. Inspect the regular polygon's equal-length, closed-chain, and point-on-construction-circle constraints before and after the final Coincident operation. Done means repeated circle-center coincidences no longer move the polygon into a degenerate configuration.

Written by the indexing model from the issue text.