Fix unicode-range tokenization and serialization

[jhaygood86]

Problem

@font-face { unicode-range: … } does not survive parsing. Four separate defects combine:

1. A start of fewer than six hex digits can't have a range end. Lexer.UnicodeRange returns immediately out of the wildcard branch, so it never reaches the -<hex> handling below it:

if (StringBuffer.Length != 6)
{
    // ... consume '?' wildcards ...
    return NewRange(FlushBuffer());   // <-- always returns
}

if (current == Symbols.Minus) { /* range end - unreachable unless exactly 6 digits */ }

U+41-5A tokenized as just U+41, leaving -5A behind as stray tokens. Only the wildcard form is mutually exclusive with the start-end form (CSS Syntax 3 §4.3.6); a - should fall through.

2. The wildcard padding loop consumes only half its wildcards. The bound 6 - StringBuffer.Length is re-evaluated each iteration while the body appends to that same buffer, so it shrinks as i grows and they meet in the middle. U+?????? stopped after three ?, leaving ??? unconsumed.

3. ValueBuilder has no case for TokenType.Range, so range tokens fall to the default arm, mark the whole value invalid, and the descriptor is dropped entirely under strict (non-tolerant) parsing — the default.

4. RangeToken serializes without its U+ prefix. Data holds the bare range, so once a descriptor is retained it round-trips as 41-5A, which is not a valid <urange>.

Fix

One change each, plus making RangeToken.SelectedRange lazy — a legal U+0-10FFFF expands to over a million strings, which the constructor materialized eagerly whether or not anything ever read the property. (Nothing in the library reads it today.)

Tests

13 tests. Tokenization.cs covers the lexer directly: start/end forms (U+41-5A, U+0-7F, U+400-4FF, U+000041-00005A), the wildcard and single-value forms (U+4??, U+??????, U+41), and on-demand SelectedRange expansion. FontFace.cs covers the descriptor end-to-end through IFontFaceRule.Range, including a comma-separated list.

10 of the 13 fail on master. The full suite (1263 existing tests) stays green, and all seven target frameworks build with no new warnings.