Professionalize TinyML packaging, core boundary, and release contract

[godofecht]

Summary

This turns TinyML's build/release surface from a development tree into an installable library contract.

• adds a dependency-free TinyML::Core target and separate TinyML::Extended target
• replaces source globbing and global compiler flags with explicit target-local CMake
• makes xsimd conditional on the extended build and GoogleTest conditional on tests/benchmarks
• adds component-aware find_package(TinyML) exports, version headers, install rules, and stable umbrella headers
• verifies the installed package from a clean downstream CMake project
• makes benchmarks, examples, and the playground opt-in
• packages releases from cmake --install into separate core/extended archives with SHA-256 checksums
• documents stable, preview, and source-only API surfaces separately from file licensing
• fixes core UB/correctness issues: uninitialized momentum, unchecked input/target/weight dimensions, global RNG mutation, unsafe weight normalization, and incorrect updateWeights() traversal

Compatibility contract

TinyML::Core is the stable semantic-versioned surface. Existing source builds still get the TinyML target for the extended library, with aliases TinyML::Extended and TinyML::TinyML. Experimental/incomplete headers remain in the repository but are not installed as supported SDK surface.

Validation

The updated CI matrix independently builds core-only GCC Release, extended GCC Release, and extended Clang Debug. Each job runs CTest, installs to a staging prefix, and builds a separate consumer against find_package(TinyML CONFIG REQUIRED COMPONENTS Core).

Closes #8.