fix: don't treat a non-prefix token before | as a namespace

[spokodev]

Problem

When a bare | introduces an empty namespace, whatever token precedes it is emitted as the namespace prefix — corrupting the selector on a round-trip:

const parser = require("postcss-selector-parser");
const rt = (s) => { let o; parser((r) => (o = r.toString())).processSync(s); return o; };

rt("/* c */|b"); // "/* c */\/\*\ c\ \*\/|b"   — the comment is re-emitted, escaped, as a prefix
rt(".a,|b");     // ".a,\,|b"                    — the comma becomes an escaped prefix
rt(".a > |b");   // ".a >  |b"                   — extra whitespace
rt(" |b");       // "  |b"

These are all valid selectors that should round-trip unchanged.

Root cause

namespace() takes the prefix from the previous token unconditionally:

namespace() {
  const before = (this.prevToken && this.content(this.prevToken)) || true;
  …
}

namespace() is reached either after a real prefix has been consumed (*|, ns|, &|, where the previous token is that prefix) or directly from combinator() when a bare | starts a compound. In the second case the previous token is a comment, comma, combinator or whitespace — not a prefix — but it was still used as one.

Fix

Only use the previous token as the prefix when it can actually be one — a word (type selector), the universal *, or the nesting & (these are exactly the tokens whose parsing hands off to namespace()). Otherwise the namespace is empty (true).

const prev = this.prevToken;
const before =
  prev &&
  (prev[TOKEN.TYPE] === tokens.word ||
    prev[TOKEN.TYPE] === tokens.asterisk ||
    prev[TOKEN.TYPE] === tokens.ampersand)
    ? this.content(prev)
    : true;

*|a, ns|a, &|foo, and |a are unchanged.

Test plan

• Added tests for an empty namespace after a comment, a comma, and a combinator (they fail on main).
• A round-trip property check over generated selectors flagged exactly these cases; the fix clears them with no regressions. Full suite green (783), eslint/tsc clean.