Expose the hard-coded center-snap threshold of Mirror editing (X/Y/Z) as a user setting

Problem When editing with mesh Mirror (X/Y/Z) enabled in Edit Mode, vertices that sit close to the symmetry plane are silently treated as "center vertices": as soon as they are transformed, they get clamped onto the plane and their left/right counterparts collapse together — even with Auto Merge disabled . The distance that decides "close to the plane" is a small hard-coded epsilon that is not exposed anywhere in the UI.

For meshes at typical human scale this is invisible. But for sub-millimeter details — narrow grooves, slits, seams, panel gaps, jewelry, miniatures, models prepared for 3D printing — the hard-coded threshold is larger than the detail itself . The attached clip shows a symmetric mesh (scene units: millimeters) with two lobes meeting at the center: simply moving the near-center vertices with Mirror X enabled welds them onto the plane and destroys the gap. Auto Merge is OFF; its threshold field has no effect on this behavior, because the center-snap uses its own internal constant.

In practice this makes Blender's mirror editing unusable for any intentionally-asymmetric-scale detail near the symmetry plane. I originally reported this as a bug ( #102961 ); it is considered intended behavior, so I'm proposing the design change here instead.

Proposal

Keep the behavior exactly as it is by default, but make the threshold controllable:

• Add a "Center Threshold" value next to the Mirror X/Y/Z toggles in the Tool Options (alongside Topology Mirror / Auto Merge), OR
• Reuse/extend the existing Auto Merge "Threshold" field for the center-snap distance, OR
• Make the epsilon relative to the scene unit scale instead of an absolute constant.

Any of these keeps the current default (existing files and workflows are 100% unaffected) while letting precision users lower the value — or set it to 0 to only ever clamp vertices that are exactly on the plane.

Why this is low-risk

• Default value unchanged → zero behavior change for anyone who doesn't touch the setting.
• The center-clamp behavior itself is useful (it keeps center seams welded); the proposal only makes its detection distance configurable.
• Comparable tools already do this: Maya exposes its symmetry tolerance as a user setting, and its topology-based symmetry avoids the positional threshold entirely.

Prior report

• Originally filed as a bug and closed as intended behavior: projects.blender.org/blender/blender/issues/102961