Add apply-and-continue shortcut for Clipping Tool

[trende2001]

Summary

Add Spacebar support to apply a clip without exiting ClipTool, matching Hammer-style workflow.

Motivation & Context

Repeated clipping is a common blockout workflow. Exiting the tool after every cut slows iteration.
This change enables fast consecutive cuts while staying in the clipping context.

Implementation Details

• Added a Space shortcut (mesh.clip-apply-stay) in ClipTool.UI.cs.
• Space now applies the current cut and keeps ClipTool active.
• Enter behavior is unchanged: apply + exit tool.
• Kept selection/target handling aligned with existing clip behavior so repeated cuts work predictably, including face-selection cases.

Screenshots / Videos (if applicable)

sbox-dev_QcI16vsQ3S.mp4

Checklist

• Code follows existing style and conventions
• No unnecessary formatting or unrelated changes
• Public APIs are documented (if applicable)
• Unit tests added where applicable and all passing
• I’m okay with this PR being rejected or requested to change 🙂