Sketcher: Small bug of help-lines and feature request

### Problem description

Some little noticed things

### Workbench affected?

Sketcher

### Steps to reproduce

See on video

### Expected behavior

I'm very glad to test out latest v26.3 and it makes me extremely happy with improvement of speed it gives for average user to do their sketch

I have few thoughts i wish to share, so dev's could see if they're reasonable to become true

1) When you try to use "help striking lines" to a point where two of them coincidence - it's not possible , but maybe could be developed in future? See Video 1

2) When you try to "catch" start from "crossing" of figures\lines - it's not possible , but maybe could be developed in future? See Video 2

3) Small bug? Or priority of snapping? When you use continuation line and get closer to a coordinate line, it prefer to snap to a second, rather a place where both meets :) See Video 3

Thank you

### Actual behavior

https://github.com/user-attachments/assets/0804a926-4438-4b73-a70f-d99990efbc64

https://github.com/user-attachments/assets/f199cb9a-1011-4e5d-b9ba-a58ef0b8789f

https://github.com/user-attachments/assets/0851d2e8-b346-41ca-aa48-55a64d2a5004

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

```shell
OS: Windows 10 build 19045
Architecture: x86_64
Version: 26.3.0dev.20260624 (Git shallow)
Build date: 2026/06/24 08:08:24
Build type: Release
Branch: (HEAD detached at e6a531c0a)
Hash: e6a531c0af11bc01d9d8aaa29ce70fa15622c3c5
Python 3.11.14, Qt 6.8.3, Coin 4.0.3, Vtk 9.3.1, boost 1_86, Eigen3 3.4.0, PySide 6.8.3
shiboken 6.8.3, xerces-c 3.3.0, Clipper2 , IfcOpenShell 0.8.2, OCC 7.8.1
Locale: Russian/Russia (ru_RU) [ OS: Russian/Russia (ru_RU) ]
Stylesheet/Theme/QtStyle: OpenDark.qss/OpenDark/
Navigation Style/Orbit Style/Rotation Mode: Blender/Trackball/Drag at cursor
Logical DPI/Physical DPI/Pixel Ratio: 96/158.196/1
```

### Last known good version (optional)

```shell

```

Start by reproducing the three Sketcher behaviors shown in the attached videos: help-line coincidence, snapping to figure intersections, and continuation-line priority. Compare each result with the expected snapping point and define separate acceptance criteria before locating the relevant Sketcher snapping code and tests.

Written by the indexing model from the issue text.