Fix Random.Shared giving identical sequences on threads started together

[SimonCropp]

Bug

The Random.Shared polyfill (below net6) keeps a [ThreadStatic] Random per thread, and created each one with new Random(). On .NET Framework the parameterless constructor seeds from Environment.TickCount, so threads that first touch Random.Shared within the same tick get identically seeded instances, and so identical sequences. Sixteen threads released together by a Barrier all produced the same four numbers.

Fix

Each thread's instance is now seeded from a single process-wide Random, read under a lock. Every thread gets a different seed regardless of when it starts.

Test

SharedDiffersAcrossThreadsStartedTogether starts 16 dedicated threads, releases them together with a Barrier, takes four values from Random.Shared on each, and asserts all 16 sequences differ. Before the fix it found 1 distinct sequence on net48. It passes on net48 (polyfill) and net10.0 (BCL).