Group compound user-defined names in inverse and power formatting

[sylvesterkaczmarek]

Fixes #471.

Group user-defined names containing *, /, or ^ when exact-name lookup is used for an inverse, square, or cube. For example, the inverse of a unit registered as m/s was rendered as 1/m/s, which parses back with different dimensions. The grouped form is 1/(m/s).

Direct registered-name output and built-in names are unchanged. No public API or parser changes. Two regression tests cover 20 compound-name round trips, direct-name preservation, and unchanged atomic-name formatting.

Validation with Apple Clang 21 and CMake 4.3.2 on macOS arm64:

• Against unchanged main (4ac3665), the compound-name regression reports 19 failed round-trip assertions; the atomic-name control passes.
• With the fix, all 19 user-defined-unit tests pass, including both new tests.
• Full build and CTest suite: 24/24 pass.
• Repository-pinned clang-format 23.1.1 checks and git diff --check pass.

Python bindings and other operating systems were not tested locally. The change is limited to the formatter's exact inverse/square/cube lookup paths.