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/TestJobSystem.cs

@@ -418,12 +418,12 @@ public class TestJobSystem
         {
             data = ref customJob,
             pExecutionFunc = &CustomJob.Execute,
-            pFreeFunc = null,
+            pFreeFunc = &CustomJob.Free,
             jobRanges = JobRanges.Single,
             priority = JobPriority.Normal,
         };
 
-        var handle = s_jobScheduler.ScheduleCustom(ref customJobDesc, false);
+        var handle = s_jobScheduler.ScheduleCustom(ref customJobDesc);
         s_jobScheduler.Wait(handle);
 
         Assert.AreEqual(1, *value);