chore: improve ryu maintenance path

[vip892766gma]

Summary:

• Add additional edge-case unit tests covering boundary float values — subnormal floats near f64::MIN_POSITIVE, the smallest positive subnormal, the largest finite f64, values just above/below powers of two, and round-trip parse-back assertions — to strengthen coverage of ryu's formatting on edge inputs without changing library behavior.
• Keep the change narrow so it is straightforward to review.

Notes:

• I kept this scoped to the relevant implementation and tests.

[dtolnay]

These tests are translated from the C++ library. If there are gaps, it needs to be sent upstream.