Closes #6321

Summary

Prior to this change, SECTION annotation endpoints had no relationship to the drawing camera border. Their positions were entirely manual — a user had to place them by hand each time, and they would not update if the drawing scale changed.

This PR introduces automatic placement of SECTION annotation endpoints: when a section annotation is created, when a drawing is activated, or when the diagram scale is changed, the two mesh vertices that form the section line are automatically positioned at a configurable inset distance (BorderOffset) from the drawing camera's orthographic border. The offset is defined in paper-space millimetres and is converted to model-space units using the drawing scale, so the same 8 mm inset looks correct at 1:50, 1:100, or any other scale.

If a user manually moves one endpoint in Edit Mode, that vertex is fingerprinted and its custom position is preserved. Only endpoints that still sit at their last auto-computed position are updated on the next activation or scale change. A dedicated operator lets the user force-reset both endpoints back to the border offset at any time.

What Changed

tool/drawing.py — new geometry helpers and core update logic

• get_camera_dimensions(camera) — derives the camera's model-space width and height from ortho_scale and the render resolution aspect ratio.
• _section_ray_rect_intersections(origin, direction, half_w, half_h) — finds the two t values where an infinite ray through the section line's midpoint intersects the camera border rectangle.
• get_section_border_positions(camera, v0_world, v1_world, border_offset_mm) — uses the above two helpers to compute the two world-space positions that sit border_offset_mm (paper mm, converted by scale) inside the camera border along the section line direction.
• update_section_endpoints(obj, camera) — the main entry point. Reads BorderOffset, AutoStartPosition, and AutoEndPosition from the BBIM_Section pset. Compares each current world-space vertex position against its stored auto-position; if they match (or no auto-position is stored yet) the vertex is auto-updated. Writes the new auto-positions and BorderOffset back to the pset, then calls bpy.ops.bim.update_representation to persist the moved vertices to the IFC geometry.
• _parse_vector3 / _format_vector3 — small static helpers to serialise and deserialise a Vector to/from a comma-separated string for IFC pset storage.

data.py — pset defaults

get_section_markers_display_data() now adds BorderOffset: 8.0, AutoStartPosition: "", and AutoEndPosition: "" to the BBIM_Section pset defaults so the values are always present and visible in the Property Sets panel even before the user changes anything.

operator.py — new operator + activation hook

• UpdateSectionEndpoints — a new bim.update_section_endpoints operator (label: "Reset Section to Border"). It clears AutoStartPosition and AutoEndPosition in the pset so that update_section_endpoints treats both vertices as unmodified, then calls update_section_endpoints to recompute. Only enabled when the active object is a SECTION annotation and a scene camera is set.
• ActivateDrawingBase._execute — after sync_references reloads the IFC geometry, iterates over all IfcAnnotation members of the drawing group with ObjectType == "SECTION" and calls update_section_endpoints for each one.

prop.py — scale-change hook

update_diagram_scale() (the callback for the diagram scale dropdown) now iterates the same drawing group and calls update_section_endpoints for every SECTION annotation in it, so endpoints are repositioned immediately when the scale is changed.

core/drawing.py — creation hook

add_annotation() now calls update_section_endpoints immediately after the new annotation is assigned to the drawing group, so a freshly created section annotation starts at the correct inset positions.

__init__.py

UpdateSectionEndpoints is added to the classes tuple so it is registered with Blender.

Notes

• Debug print("[SECTION] …") statements are present throughout and should be removed before final merge.
• AutoStartPosition / AutoEndPosition are stored in world space in this version. An object-level translate (moving the annotation object without editing vertices) will change world-space vertex positions while leaving local-space v.co unchanged, which can cause false "manually moved" detection. This is a known limitation of the current approach.
• BorderOffset is intentionally exposed in the Property Sets panel rather than a custom UI panel, so users can inspect and override it without any additional UI work.

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