feat: external linear algebra libraries integrations + quantity::magnitude()

[mpusz]

Summary

Adds opt-in integrations that let mainstream linear algebra libraries (Eigen, GLM, Blaze) act
directly as quantity representation types, plus a quantity::magnitude() member for vector
quantities. Resolves #301.

Linear algebra integrations

• New opt-in adapters, available both as headers (mp-units/integrations/{eigen,glm,blaze}.h)
  and named modules (mp_units.integrations.{eigen,glm,blaze}).
• Each header is guarded with __has_include, so it is a harmless no-op when its library is
  absent. The adapters only wire up the representation customization points. No user adapter code
  is needed, and the libraries' native vector and matrix types are used directly.
• The built-in cartesian_vector needs no plugin, and plain double and int work as
  one-dimensional vector representations.

representation_canonical_type customization point

Materializes expression-template results (for example the Eigen and Blaze lazy proxies) into
their concrete type before they are stored in a quantity, so a quantity never holds a
dangling reference to an expired operand.

quantity::magnitude() member

• A vector quantity now supports magnitude() directly (the magnitude CPO finds the member),
  returning the Euclidean magnitude as a scalar quantity. This removes the need for user-defined
  magnitude_of helpers.
• Known V2 limitation, documented in the code and the user guide: the result drops the precise
  quantity spec down to the unit's kind, because V2 cannot express a dedicated scalar-magnitude
  quantity spec. For the same reason, quantity-level scalar_product() and vector_product()
  are not provided. They would need to return a different quantity kind, which V2 references
  cannot name, so they remain available only on the raw representation.

Packaging

Per-backend CMake targets mp-units::integrations-<name> are provided for source consumers
(add_subdirectory / FetchContent) and raw cmake --install. They are intentionally not
surfaced as Conan components (documented in conanfile.py): Conan consumers link
mp-units::integrations plus their own linear algebra library.

Docs and tests

• User guide (representation_types.md), the custom-representation how-to guide, and a new
  annotated example (docs/examples/linear_algebra.md with example/linear_algebra.cpp)
  compiled against all backends in both header and module modes.
• test/runtime/linear_algebra_test.cpp reworked to exercise the integrations and the
  compile-time guarantees.
• test_package extended with header-mode integration consumers for each backend.

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