Serialize BCL values without reflection

[Eli Pinkerton (wallstop)]

Why: Common framework values still fell back to reflection under IL2CPP.

What:

• Add compatible DateTime, TimeSpan, Guid, and decimal formats
• Cover roots, members, collections, maps, and generic contracts
• Verify both protobuf-net oracle versions and Unity 6000

Refs #399
Refs #343

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Note

Medium Risk
Changes serialization wire compatibility and facade resolution for common save-game types; behavior is heavily tested against protobuf-net but any custom DateTime formatter override must still satisfy root-marshal rules.

Overview
Adds reflection-free WallstopProto support for DateTime, TimeSpan, Guid, and decimal, using protobuf-net’s BCL wire shapes so bytes match both oracle majors (2.4.9 and 3.2.56).

Runtime: New WProtoBcl formatters plus root marshals register with the existing built-in bootstrap. WProtoFacade routes BCL roots through the root wrapper; generic/member paths treat these as last-wins scalars (not merged sub-messages) and omit default TimeSpan/Guid/decimal like protobuf-net, while still writing DateTime.MinValue when needed.

Generator: Shape resolves the four types via WProtoFormatterProvider; DateTimeOffset stays WPROTO003. Map keys may use the supported BCL types.

Tests & docs: BclDifferentialTests and expanded generator contracts pin oracle parity, golden hex, map keys, malformed payloads, and generic Box<T> closures; changelog and serialization docs list the new built-ins.

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