Replace snprintf("%.*f") with integer-based formatting for finite
float values with accuracy_decimals 0-3 (covers virtually all sensor
usage). Falls back to snprintf for higher accuracy or NaN/Inf.
Uses lrint() with double cast for the multiply to match snprintf's
rounding behavior exactly. The fast path avoids snprintf's heavy
float formatting machinery entirely.
Also optimizes value_accuracy_with_uom_to_buf to append the UOM
string directly instead of going through snprintf.
Adds C++ unit tests that verify output matches snprintf for a range
of values including edge cases.
Benchmark: 92,961ns -> 6,484ns (14.3x faster, 2000 iterations).
J. Nick Koston