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fix(dfixpnt): floor and ceil add a zeroed unit, not an uninitialized one (#1502)

floor() of a negative value with a fraction subtracts one unit, and ceil() of a positive one
adds it. Both built the unit as a default-constructed dfixpnt with one digit set. dfixpnt is
trivially constructible, so every other digit and the sign were indeterminate, and the
result depended on whatever was on the stack (#1476). For
dfixpnt<8, 3, BCD, Modulo, uint8_t>, floor(-12.375) came out as -46513 and ceil(12.375) as
46513 with gcc at -O2, and clang crashed inside floor(). Each function was wrong on half its
domain.

The unit now comes from UnitOf(), which zeroes the value first and then sets the units
digit. A type with no integer digits (radix == ndigits) cannot hold 1, and setdigit(radix,
1) wrote past the value there; with Modulo arithmetic 1 wraps to 0 anyway, so for those
types the unit is 0 and floor and ceil return the integer part, 0.

The new static/fixpnt/decimal/math/floor_ceil.cpp, in a new math folder of the dfixpnt
tests, builds each value from a scaled integer n and checks floor and ceil against integer
floor and ceiling division of n by 10^radix. It covers the BCD, BID and DPD encodings,
Modulo and Saturate arithmetic, and 8- to 64-bit limbs. Level 1 is exhaustive over every
value of dfixpnt<4,2> (both modes), <4,1>, <3,0> and <3,3>, plus the reported cases, and
samples <8,3>, <12,5> and <16,8>.

Verification: against main the test segfaults at -O2 on gcc 13.3 and clang. With the fix,
levels 1-4 pass on both, and all 15 dfixpnt and blockdecimal tests pass on both. The fix
also holds with -ftrivial-auto-var-init=pattern, which fills uninitialized stack memory with
a fixed pattern.

test(dfixpnt): check the no-integer-digit floor and ceil before the range filter

CodeRabbit on #1502. For a type with no integer digit, such as dfixpnt<3,3>, the reference
floor of a negative fraction and ceil of a positive one is -1000 or +1000, outside the
type's [-999, 999... (continued)

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