Add decibel gain and absolute level types

[cleishm]

Summary

Adds an opt-in <electro/decibel> header with two types, modeled on the duration/time_point distinction:

• gain — a dimensionless logarithmic ratio (dB). An ordinary quantity unit: adds, subtracts, negates, scales.
• level — an absolute point on a logarithmic scale (dBm, dBW, dBV, dBmV, dBµV), parameterized by a reference tag that carries the linear unit, the 0 dB reference precision, the 10·log10 vs 20·log10 factor, and the display symbol.

The affine algebra is enforced at compile time:

auto rx = 20_dBm + 6_dB - 2_dB - 90_dB;  // level ± gain → level: -66 dBm
auto margin = rx - (-100_dBm);           // level − level → gain: 34 dB
// rx + rx;   // ill-formed
// rx * 2;    // ill-formed

Crossing into the linear domain is via explicit named functions — to_linear(30_dBm) is 1000.0 mW, to_level<dbm>(milliwatts(1)) is 0 dBm — the only non-constexpr parts of the header (std::log10/std::pow aren't constexpr until C++26). Combining uncorrelated signals is the explicit add_powers() (10 dBm ⊕ 10 dBm = 13.01 dBm), restricted to power references since summing linear amplitudes on a field reference would model coherent addition instead.

electro.hpp is unchanged; decibels are opt-in so <cmath> and the non-constexpr bridge stay out of the core header.

Version

Bumped to 0.2.0. The vcpkg port SHA512 is reset to the placeholder — it can only be set once the v0.2.0 release tarball is published, mirroring the two-step flow used for v0.1.0.

Testing

• 21/21 tests pass (Catch2), including ~150 lines of static_assert compile-time checks and concept-based ill-formed-operation tests with positive controls.
• Every README snippet compile-verified.
• Local coverage is Apple Clang only; GCC 14 / Clang 17 / MSVC come from CI. Watch the _dBµV literal on MSVC (needs the already-required /utf-8).