Port shearing-sheet UI and runtime updates from aarch64

[Francyrad]

This PR ports the full set of shearing-sheet and GUI/runtime improvements developed and validated in the Apple Silicon tree back to the x86 devel branch.

Main changes:

• port shearing-sheet runtime support, SEI-related updates, gravity/collision helpers, diagnostics, tests, and setup/preset wiring
• port GUI features added during validation on Apple Silicon: live plot refresh toggle, SI plot labels, SPH visibility fixes, ruler and particle-to-particle measurement tools, center-view button, palette autoscale, total-mass readout, and shearing-box overlay
• port shearing-sheet IC improvements: total-mass+count fill mode, mass-unit selector, box-size units, and related UI cleanup

Notes:

• this is a source-only port from the working Apple Silicon tree into the x86 devel source tree
• I intentionally did not build in this branch while preparing the sync, because the requested deliverable here was the clean source port and PR preparation

Clean source copies without build artifacts were also prepared locally outside the repository for both x86 and aarch64 trees.