[tamnd]

Part of #2492.

PostgreSQL casts a real to numeric through its text with 6 significant digits (float4_numeric prints with FLT_DIG), and a double precision with 15 (DBL_DIG). rudb widened a real to a double and printed it with 15 digits, so 16777216::float4::numeric gave 16777216 and (1.0/3.0)::float4::numeric gave 0.333333343267441. PostgreSQL gives 16777200 and 0.333333.

Numeric::from_f32 now prints the float with 6 significant digits, and Numeric::from_f64 and it share the code that reads the digits back. The PostgreSQL numeric cast in rudb-kernels calls the one that matches the type of the value. The pin has no numeric type, so this does not change it.

A unit test in rudb-pgtypes checks the oracle answers for large, small, negative zero, rounded and infinite values. The scratch server gives the same answers as the PostgreSQL 19 oracle for the casts, and for stddev, var_samp, sum and avg over b::numeric where b is a real. On the harness the statements score goes from 1862 to 1866 of 2588 (all four in aggregates), and nothing regresses.