Add per-session seeds and an own deterministic random generator

[holonsoftFounder]

Summary

• x.UseSeed(seed) per factory (default stays 1337), CreateSession(recursionLimit, seed) per session.
• Every data source gets its own stream derived from the session seed and Type.Member (SeedDerivation, FNV-1a), applied once via ISessionSeedable; an explicit SetSeedToRandomValue in the configuration lambda wins. Two members with the same source type no longer correlate, nullable value type sources no longer go null together, a member's values do not depend on which other members exist. Null evaluators of RespectNullableAnnotations, method argument sources, type factories (constructor arguments) and the Random(count) shuffle follow the seed.
• StableRandom: xoshiro256** seeded through SplitMix64, a Random subclass with every public member overridden, used by DataSourceBase, DefaultRandomNullEvaluator and CollectionContext. The sequence of a seed is defined by our code, not by the runtime. Output pinned by tests, uniformity and range tests included.
• Breaking: AutoPocoGlobalSettings removed (mutable process-wide state), AutoPocoDefaults holds the constants Seed, NullCreationThreshold, RecursionLimit.
• Breaking: every generated sequence changed once. All stable-sequence tests regenerated. The shared helper NextReturnsStableElementListInTermsOfTestability now appends the actual values as a ready-to-paste C# literal (CSharpLiteral) to the failure message.
• Three pre-existing tests only passed by luck of the old stream: two waited at most 21 draws for a 15 percent null, one expected millisecond 999 in 300 draws over a 1000 millisecond window. Fixed with sound bounds.
• Not done on purpose: a factory-wide default null threshold. The Nullable* constructors chain through plain int parameters, so "explicit 15" and "default 15" cannot be told apart. Per-source thresholds and RespectNullableAnnotations(threshold) cover it.

Test plan

• 38 new tests: StableRandom pinned output, ranges, uniformity, rejection path, span/array parity, shuffle; SeedDerivation reference values; DataSourceBase session seed semantics; integration for factory seed, session seed, cross-factory equality, per-member independence, nullable independence, explicit seed precedence, shared generation-time source, deterministic Random(count), constructor arguments, nullable annotation patterns
• dotnet build -c Release zero warnings
• 797 tests green per framework on net8, net9, net10 (Windows)
• Two separate processes with seed 2024 print identical data, seed 7 differs

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