Soft player collision: pass-through movement + push-apart separation - #15

What

Replaces hard player-vs-player body-blocking with soft collision (on by default, sv_player_softcollision 0 restores stock behavior):

• Pass-through: new MoveFilter.NoPlayers — clips exactly like Normal except player bodies (SOLID_SLIDEBOX + FL_CLIENT) never block. All player movement-hull traces in PlayerPhysics (main slide/PushEntity, ground downtrace, ramp check, crouch stand-up, IsFlying) route through a cvar-gated PlayerClipFilter(). Running through someone at speed is a clean slip; you can deliberately walk through a standing player.
• Push-apart: new server-side PlayerSeparation pass (first step of GameWorld.OnEndFrame, after all players have moved). Overlapping hulls slide apart positionally — horizontal-only, split symmetrically, at sv_player_softcollision_pushspeed (default 150 u/s per pair) with each step capped at the remaining penetration, so bodies ease out and settle exactly side by side (no jitter at contact, no fling).

Why

Requested gameplay change for Vortex Arena: fast crossing paths should have minimal interaction; standing overlap should gently resolve to adjacent positions; body-blocking should be impossible.

Reviewer notes

• Weapons unaffected: only movement traces use the new filter — hitscan/melee/projectile traces keep their existing filters, so players stay fully shootable. Monsters (SLIDEBOX + FL_MONSTER) still block movement.
• Prediction stays in lockstep: sv_player_softcollision rides the MoveVarsBlock wire (v9, slot 50, append-only; AbsentDefaults carries the unset→ON semantics), read ambiently by PlayerPhysics like sv_gameplayfix_nudgeoutofsolid. The separation push itself is server-only/unpredicted — at ≤75 u/s per body the reconcile smoothing absorbs it.
• Separation is positional, not an impulse: ground friction (~800 u/s² at rest) would eat any reasonable velocity-based push; a trace-guarded origin slide sidesteps that and stops naturally at contact.
• Guard rails: pushes are trace-guarded against world geometry (NoPlayers filter so a third body can't wedge the push), never vertical, skip dead bodies/spectators, keep frozen statues immobile (free body absorbs the full push), and use a deterministic pair-keyed direction for fully co-located bodies (no RNG stream advances). Moved bodies are relinked into the entity area grid.
• Incidental fix: docs/reference/CVARS.md had committed merge-conflict markers inside the generated inventory — cleaned up during the regen (which also picks up the two new cvars).
• Tests: new PlayerSoftCollisionTests (filter skip vs players/monsters, settle-adjacent, symmetric midpoint, one-tick deflection bound, cvar off, dead-body exemption); SnapshotDeltaTests wire-count pin updated 49→50. Full suite: 2991 green.
• Not yet playtested in-game — worth a bot match to tune sv_player_softcollision_pushspeed by feel.

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