Do builders stay away from building a frame 55.5 cm (c-t-c) instead of my 56 cm size?
My seat angle is 72.8 degrees, so a 55.5 cm frame would be a little over 73 degrees instead of a little under.
The half centimeter height difference at the head tube can be made up with a spacer at the headset.

no, even number sizes are a production thing, and the factories might not even make bikes the size their marketers say. Take a tape measure to a bike shop and see.

But also, size is independent of angle and vice versa. I can make a 54.321cm frame with 73 degree angles, no problem. Are you using a frame design program that only changes one thing at a time or something?

Seat tube angle is the resultant figure of placing the saddle at the desired amount of setback from the bottom bracket, at the desired saddle height. It is not a number that you choose and then design a frame to make it work.

The OP has an obsession over using the nominal angle of lugs.

Custom builds they will do as you wish, and pay for.

One could build, or get built anything that one wants. However, I'm not convinced it would make any difference, or that a person could tell the difference between a 55cm frame and a 55.5cm frame, or the difference between a 55.5 cm frame and a 56cm frame.

Get your seat set right, and then adjust your handlebars as you wish.

Note, most stems are also made in 1cm increments.

Specialized makes stems with adjustable angles, but most of that can also be accomplished by more or fewer spacers to achieve the same place where the bar mounts to the stem.

Observed more than one Italian builder stamp a frame with a .5 cm notation as part of the size...
"it's CUSTOM, for YOU"
Marketing.

FWIW, Andy Hampsten's bike he rode over the Gavia pass and to many other victories back in the late 80's was exactly 55.5 cm (C-C).

Rider's height: 1.75m (5' 9")
Rider's weight: 62kg (136lb) (during '88 Giro d'Italia)
Saddle height, from BB (c-t): 753mm
Saddle setback (ask mechanic): 48mm
Seat tube length, c-t: 565mm
Seat tube length, c-c: 555mm
Tip of saddle nose to C of bars (next to stem): 570mm
Saddle-to-bar drop (vertical): 85mm
Head tube length: 140mm
Top tube length: 565mm
Total bicycle weight: 8.93kg (19.69lb), w/o pedals

Historic Pro Bike: Andy Hampsten's 1988 7-Eleven Huffy Giro d'Italia | Cyclingnews.com

Interestingly, to me anyway, Andy showed up to one of the Eroica rides on this same bike last year. That man is ageless!

Bikes of the Stars: Andy Hampsten's 1988 Giro bike

I got my first custom bike because the top tube length on the closest production model to what I needed, according to my fitting, was 3cm too short with the longest available stem. The bike I was riding at the time had a 14cm stem, long reach bars, and was still 5cm too short.

So not even close. The only dimension on the bike I fret down to .5cm is the fork offset, and sometimes the stem. I have had a couple custom stems to tidy things up.