Add format-spec rendering in display units (0.3.0)

[cleishm]

Summary

Mirrors thermo-cpp's new formatter-side rendering (cleishm/thermo-cpp#5): a non-empty format spec on a quantity renders the value in display units as a double, honoring the full floating-point spec syntax. The empty spec keeps today's exact-count form unchanged.

std::format("{:.1f}", millivolts(3300));      // "3.3V"
std::format("{}", millivolts(3300));          // "3300mV" (exact stored value, unchanged)
std::format("{:.1f}", milliampere_hours(200)); // "0.2Ah"
std::format("{:.1f}", 1050_cdB);              // "10.5dB"
std::format("{:.1f}", 1301_cdBm);             // "13.0dBm"

• SI-prefixed precisions render in the base unit ("3.3V"); conventional non-decade precisions (mAh/Ah, Wh/kWh) render in their conventional unit via a new internal _display_unit trait ("0.2Ah", never "720.0C") — analogous to thermo's fahrenheit display family.
• Decibel gains and levels render their value in dB with the reference symbol appended.
• Non-empty specs were previously compile errors, so the change is purely additive; version bumped to 0.3.0 (also syncs the Doxyfile version, which was stale at 0.1.0).

Test plan

• New spec-rendering test cases for quantities (V/A/Ω, Ah/Wh families, width pass-through) and for gains/levels
• Existing empty-spec format tests pass unchanged
• Non-format build path verified: -DCONFIG_ELECTRO_STD_FORMAT=0 -fsyntax-only
• 23/23 tests pass