Fix three CSS compressions that break valid stylesheets

[marevol]

Three CssCompressor defects that break valid stylesheets: one changes which elements a rule selects, one aborts the build, one makes the browser drop a declaration.

1. A descendant combinator was deleted before a pseudo-class

@media screen{p :link{...}}   ->  @media screen{p:link{...}}
.a{& :hover{...}}             ->  .a{&:hover{...}}
@supports (display:grid){div :hover{...}}  ->  {div:hover{...}}

p :link is links descended from a p; p:link is a p that is itself a link. They select disjoint sets of elements — the rule ends up applying to the wrong ones.

The colon-protection pattern was anchored at ^ or after a }, so a selector that is the first thing inside a block had no anchor and the space in front of its pseudo-class was eaten. That covers the first rule in every @media / @supports / @layer / @container — and, with CSS nesting, every nested rule, which is by definition the first thing after a { or a ;. A rule preceded by a sibling's } was protected, which is why the top-level golden fixtures never showed it.

{ and ; are now anchors too, and the anchor is a lookbehind rather than part of the match: an at-rule's own prelude ends with the very { that opens the block, and consuming it left the first nested rule with no anchor of its own. ; and } are excluded from the runs on either side of the colon so a match cannot start in a declaration block and reach into the next selector — that is what keeps color:red out of the protection, where it would only cost bytes.

2. rgb() with CSS Color 4 channels aborted the build

$ echo 'a{color:rgb(0 0 0)}' > x.css && yuicompressor --type css x.css
[ERROR] x.css: For input string: "0 0 0"        exit 1, no output at all

The character class [0-9,\s] admitted whitespace, so split(",") yielded the single token "0 0 0" and Integer.parseInt threw NumberFormatException — unchecked, and not declared by compress(Writer,int) — on input every browser accepts. A Maven build fails on it.

The pattern now matches only the legacy comma form, which is the one that can be shortened to hex. The modern form is left alone: that costs bytes, not the build. Channel digits are bounded so a nonsense value cannot overflow parseInt either; rgb(1000,500,300) still clamps to #fff as the golden expects.

3. Operator spacing was only repaired inside calc()

a{width:min(10px + 5px,5px)}  ->  a{width:min(10px+5px,5px)}

CSS Values 4 requires whitespace on both sides of + and - in a <calc-sum>, which is what every math function takes — so the browser drops the declaration. respaceCalcOperators searched for the literal "calc(" and never saw min, max, clamp, round, mod or rem.

It now matches any of them, on the whole function name so minmax() is not caught by the min entry, and still on a vendor prefix so -webkit-calc() keeps working. The nested-function and custom-property hardening from #9 and #11 comes along unchanged.

Verification

• 143 real-world stylesheets: all still compress, all still byte-identical on a second pass, and the set of declarations that the CSS grammar rejects after compression (checked with css-tree's lexer) is identical to before this branch — no new corruption.
• CssStructureTest (new, 12 cases) pins each defect together with what must not change: a declaration's own colon still loses its spaces, a pseudo-class with no space stays tight, comma-separated rgb() still shortens, minmax() is left alone, margin: 0 -5px is untouched.

Full suite: 777 tests, 0 failures, 3 skipped.

Independent of #13, #14 and #15; all branch from main. #13 also touches CssCompressor but a different region.