Implement cent/ucent

[rikkimax]

For this we can thank DeepSeek v4 flash.

[github-actions]

DMD perf check

Metric	Base	PR	delta
compile hello.d (instr)	215.6 M	218.0 M	+1.14%
compile hello.d -O (instr)	234.1 M	236.6 M	+1.07%
compile Phobos (instr)	5,110.5 M	5,162.3 M	+1.01%
compile Phobos codegen (instr)	1,472.2 M	1,483.7 M	+0.78%
compile vibe.d (instr)	15,079.5 M	15,190.3 M	+0.73%
dmd binary size (stripped)	6.92 MB	7.01 MB	+1.30%
hello binary size	0.72 MB	0.72 MB	0.00%
peak RSS (compile hello.d)	43 MB	44 MB	+0.33%
peak RSS (compile Phobos)	617 MB	618 MB	+0.19%
peak RSS (compile vibe.d)	1916 MB	1918 MB	+0.10%

[rikkimax]

Mir, Mir-random, Phobos have got code that has been silently "supporting" (u)cent that is now erroring.

[rikkimax]

This is ready for a first pass over it @WalterBright, we can talk more at meeting about it.