Accept whitespace around the offset sign in :nth-child(An+B)

[jhaygood86]

Problem

:nth-child(10n + 1)   /* rejected */
:nth-child(10n+1)     /* accepted */

CSS Syntax 3 §6.2 defines <a-n-plus-b> with, among others, the production:

<n-dimension> ['+' | '-'] <signless-integer>

and §6.1 states:

Whitespace is permitted on either side of the + or - that separates the An and B parts when both are present.

listing 3n + 1, +3n - 2 and -n+ 6 as valid examples.

The two spellings lex differently:

Source	Tokens after 10n
10n+1	one signed <number> +1
10n + 1	<whitespace>, <delim +>, <whitespace>, <number> 1

ChildFunctionState.OnOffset handles only Whitespace and Number, so the standalone delim falls through its default: arm into OnBeforeOf — which is looking for of or ) — and sets _valid = false, dropping the rule.

This affects :nth-child(), :nth-last-child(), :nth-of-type() and :nth-last-of-type() alike.

Fix

Handle the +/- delim in OnOffset and add an AfterOffsetSign state for the digits that follow.

That state is deliberately strict rather than just setting a sign flag and staying put, because the production calls for a <signless-integer>, which §6.2 defines as:

a <number-token> with its type flag set to "integer", and no sign character

So a second sign stays invalid — and §6.1 lists 3n + -6 among its invalid examples, which is exactly this case. NumberToken.Data preserves the authored sign character, which is what the check keys on.

Tests

20 theory cases in SelectorsTests.cs, asserting ChildSelector.Step/Offset rather than round-tripped text:

• spaced forms — 10n + 1, 10n - 1, 2n + 0, -2n + 3, and extra internal whitespace
• compact forms and odd/even, which must keep working unchanged
• the three other :nth-* variants
• the spec's own invalid examples — 3n + -6, 3 n, + 2n, + 2 — plus 10n + +1, 10n + + 1 and 10n + n

8 fail on master; the invalid cases pass both before and after, confirming the fix doesn't loosen the grammar. The full suite (1263 existing tests) stays green with no existing assertion modified, and all seven target frameworks build with no new warnings.