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.Test/UnitTest/Jobs/TestJobs.cs

@@ -102,7 +102,7 @@ public unsafe struct IncrementJob : IJob
     }
 }
 
-public unsafe struct CustomJob
+public unsafe struct CustomJob : ICustomJob<CustomJob>
 {
     public int* value;
 
@@ -110,4 +110,9 @@ public unsafe struct CustomJob
     {
         *job.value += 1;
     }
+
+    public static void Free(ref CustomJob job)
+    {
+        // No resources to free in this example.
+    }
 }