Generic to_string/formatters with format-spec rendering in hertz (1.2.0)

[cleishm]

Summary

Mirrors thermo-cpp's formatter generalization (cleishm/thermo-cpp#5): the six per-type to_string overloads and six std::formatter specializations are replaced by one generic template each, covering every frequency<Rep, Precision> instantiation (including user-defined ones like frequency<double, std::mega>). An empty format spec keeps today's exact-count form; any floating-point spec renders the value in hertz:

std::format("{}", kilohertz(433));        // "433kHz" (exact stored value, unchanged)
std::format("{:.1f}", millihertz(1500));  // "1.5Hz"
std::format("{:.2f}", millihertz(1534));  // "1.53Hz"

• New internal _si_prefix trait maps precision ratios femto→tera; an unmapped precision with an empty spec throws format_error from parse() (a compile error for literal format strings), while explicit specs work for any precision.
• Output strings are byte-identical for all existing typedefs; non-empty specs were previously compile errors, so the spec path is purely additive.
• Version synced to 1.2.0 across CMakeLists.txt, idf_component.yml, docs/Doxyfile, and vcpkg-port/vcpkg.json (they had drifted: 1.1.2 / 1.1.3 / 1.0.0 / 1.0.0).

Test plan

• New test pins all six empty-spec output strings byte-for-byte (previously untested)
• New spec-rendering tests including width pass-through
• to_string coverage extended to terahertz
• Non-format build path verified: -DCONFIG_FREQUENCY_STD_FORMAT=0 -fsyntax-only
• 43/43 tests pass