Part of kcenon/common_system#454

What

Update the compiler requirements in README.md to reflect that database_system transitively requires GCC 13+/Clang 17+ through its thread_system dependency (via ecosystem feature).

• Current: README documents GCC 11+/Clang 14+ as minimum compiler requirement
• Expected: README documents GCC 13+/Clang 17+ with a note explaining the transitive dependency
• Scope: README.md compiler requirements section only

Why

• database_system's ecosystem feature enables thread_system integration
• thread_system requires GCC 13+/Clang 17+ (higher than baseline)
• Users building with the ecosystem feature using GCC 11/12 will encounter build failures
• Accurate documentation prevents wasted debugging time

Where

• README.md — Requirements/Prerequisites section listing compiler versions

How

Technical Approach

Update the compiler version row in the requirements table:

| C++20 Compiler | GCC **13+** / Clang **17+** / MSVC 2022+ / Apple Clang 14+ | Yes |

Add explanatory note:

> **Note**: When using the `ecosystem` feature (which enables thread_system integration), compiler requirements are GCC 13+/Clang 17+. Base database_system without ecosystem feature requires GCC 11+/Clang 14+.

Acceptance Criteria

• README clarifies compiler requirements for ecosystem vs base usage
• Note explains the conditional requirement from thread_system
• Consistent with actual build requirements