SPMD API overhaul: gather/scatter, job & packaging updates

- ISPMDLane: add MaskGather, MaskStore, Scatter, MaskScatter; update MaskLoad/Gather signatures for hardware parity
- WideLane/ScalarLane: implement new methods with HW/fallback logic
- MathV: gather/mask-gather now delegate to lane methods
- Vector2/3/4: add CompressStore, Scatter, MaskScatter
- SPMD jobs/tests/README: migrate to new APIs for correctness
- Use Unsafe.BitCast instead of Unsafe.As/AsRef
- Add SPMDUtility for gather index extraction
- Job system: add ICustomJob<TSelf>, ScheduleCustom overload
- FreeList concurrency obsolete; always thread-safe
- NuGet: include LICENSE/README, set license/readme in .csproj
- Docs: update SPMD usage, clarify safety notes
- Minor: doc fixes, CompressStore test improvements

Misaki.HighPerformance.Mathematics.SPMD/Templates/WideLane.tt

@@ -33,9 +33,7 @@ public readonly unsafe partial struct WideLane<TNumber> : ISPMDLane<WideLane<TNu
 <# foreach (var c in conversions) { #>
         if (typeof(TNumber) == typeof(<#= c.From #>) && typeof(TOtherNumber) == typeof(<#= c.To #>))
         {
-            ref var vFrom = ref Unsafe.As<Vector<TNumber>, Vector<<#= c.From #>>>(ref Unsafe.AsRef(in value));
-            var vTo = <#= c.Method #>(vFrom);
-            return Unsafe.As<Vector<<#= c.To #>>, TOther>(ref vTo);
+            return Unsafe.BitCast<Vector<<#= c.To #>>, TOther>(<#= c.Method #>(Unsafe.BitCast<Vector<TNumber>, Vector<<#= c.From #>>>(value)));
         }
 
 <# } #>