Add mp_knife_wall_sparks cvar: spark effect when a knife hits a wall

[belomaxorka]

What

New cvar mp_knife_wall_sparks (default 0 — original behavior). When enabled, a spark effect (TE_SPARKS) is shown at the point of impact whenever a knife hits a wall — both for the slash (CKnife::Swing) and the stab (CKnife::Stab) attack, in the exact code branches where weapons/knife_hitwall1.wav is played.

Why

This is a popular cosmetic tweak that servers currently implement with AMXX plugins that hook every EmitSound call and reconstruct the impact point indirectly (via get_user_aiming and distance heuristics), which is both inaccurate and wasteful. Native support is a one-liner per attack path using the existing UTIL_Sparks() utility:

• the exact impact point is taken from the attack TraceResult;
• the effect is sent as MSG_PVS (only to nearby players), not MSG_ALL;
• zero overhead when the cvar is disabled.

Implementation notes

• Gated by #ifdef REGAMEDLL_ADD, vanilla builds are unaffected.
• Default 0 keeps the original behavior bit-for-bit.
• Documented in README.md and dist/game.cfg.

Tested

Verified on a live ReHLDS server (Windows, CS 1.6): with mp_knife_wall_sparks 1 sparks appear at the exact impact point for both attack buttons; with 0 behavior is identical to the original (sound only). The cvar can be toggled at runtime.