docs: add .NET 10 / C# 14 migration plan for post-Unity 6.8 CoreCLR transition

Plan to lift Sharpy.Core from netstandard2.1/C# 9.0 to net10.0/C# 14 across
6 phases: foundation (multi-target), Core modernization (BCL upgrades), codegen
modernization (collection expressions, primary constructors, record structs),
language features (compound assignment, extension members, list patterns, generic
math), Span-based performance, and Unity integration.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: antonsynd