Type-erase Quantity text conversion - #217

Summary

• type-erase Quantity text formatting into floating-point and integral backends
• type-erase parsing and runtime unit-scale conversion
• preserve templated hard-unit arithmetic so compile-time units and scale factors still fold
• format signed and unsigned integer quantities exactly instead of routing them through double

Why

Quantity!(T, unit) previously instantiated the complete formatting, parsing, and dynamic scale-conversion machinery for each representation and unit. These are cold runtime operations and gain nothing from hard-unit specialization. Thin typed adapters now pass the value and unit into shared backends, while arithmetic between hard units remains specialized.

An isolated SmartEVSE ESP32 release A/B build reduced the linked image by 416 bytes. That firmware emits only one Quantity formatter and three parsers, so targets using more Quantity text instantiations should gain more deduplication.

Validation

• uRT Windows unittest configuration builds successfully
• focused urt.si.quantity suite passes, including hard-unit arithmetic, scaled integer round-trip, long.min, values beyond exact double precision, and ulong.max
• SmartEVSE ESP32 firmware links successfully; its existing partition-size check still fails after producing the image
• git diff --check passes