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.LowLevel/Buffer/FreeList.cs

@@ -15,6 +15,7 @@ public unsafe struct FreeList : IMemoryAllocator<FreeList, FreeList.CreationOpti
     {
         public nuint alignment;
         public nuint chunkSize;
+        [Obsolete("Max concurrency level is no longer used and will be ignored. FreeList is now designed to be thread-safe without a fixed concurrency level.")]
         public int maxConcurrencyLevel;
     }
 
@@ -501,7 +502,6 @@ public unsafe struct FreeList : IMemoryAllocator<FreeList, FreeList.CreationOpti
     /// <remarks>
     /// This is thread safe.
     /// </remarks>
-    [MethodImpl(MethodImplOptions.AggressiveInlining)]
     public void* Allocate(nuint size, nuint alignment, AllocationOption allocationOption = AllocationOption.None)
     {
         if (_disposed != 0)
@@ -605,7 +605,6 @@ public unsafe struct FreeList : IMemoryAllocator<FreeList, FreeList.CreationOpti
     /// <remarks>
     /// This is thread safe.
     /// </remarks>
-    [MethodImpl(MethodImplOptions.AggressiveInlining)]
     public readonly void Free(void* ptr)
     {
         if (_disposed != 0 || ptr == null)