docs(codec): sync CoreCLR-first compatibility contract

[SunSi12138]

Promote the reviewed codec compatibility documentation from dev to main.

• Core compatibility matrix is explicitly CoreCLR.
• Six desktop .NET 10 CoreCLR identities remain the release-gated 6x6 matrix.
• Browser/Android/iOS CoreCLR are documented as scoped evidence-backed extensions.
• Mono is auxiliary evidence only, with compatibility described by concrete type/layout categories.
• AutoLayout, pointer-width-sensitive native integers, framework raw semantic layouts, and canonical padding bytes are explicitly outside a general Mono/CoreCLR raw ABI promise.

Documentation only. No production code, compatibility policy, version metadata, tags, releases, or package publication changes.