docs, rewrite README with schema-drift prevention positioning and build-system integration

[steven-varga]

Context

Following a product strategy review, the compiler's value proposition has been reframed from "boilerplate saver" to "schema-drift prevention tool." The README must reflect this.

Input materials

• Value prop (Mary): The compiler is a build-time correctness gate that guarantees HDF5 compound types match C++ structs exactly — preventing silent dataset corruption when structs evolve.
• Positioning (John): "h5cpp-compiler keeps your HDF5 schemas in sync with your C++ structs — automatically, at build time, before silent corruption becomes a runtime bug."
• Marketing strategy (John): Lead with correctness, not LOC saved. Target scientific/simulation maintainers who have been burned by schema drift. Package as build-system hygiene tool.

Deliverables

• Reframe opening paragraphs around schema synchronization / correctness boundary
• Add "Do I need this?" decision tree (simple structs vs. complex nested structs)
• Add 30-second copy-paste demo showing struct change → compiler catch
• Add CMake integration example (pending separate CMake issue)
• Remove "source code transformation tool" jargon
• Add compatibility matrix with h5cpp library versions
• Add install instructions for package managers (conan, vcpkg aspirations)

Must NOT include

• Benchmarks of lines-of-code saved
• "AI-powered" or "seamless" buzzwords
• Corporate positioning speak