fix: [#2088] Handle CSS hex escape sequences in querySelector

[coffeeandwork]

querySelector and querySelectorAll throw a SyntaxError (or silently return null) when the selector contains CSS hex escape sequences like #\39 7e356d3-..., which is what CSS.escape() produces for IDs starting with a digit.

The root cause was in SelectorParser:

1. SELECTOR_GROUP_REGEXP wasn't escape-aware — it saw the trailing space in \39 as a descendant combinator and split the selector in half.
2. SELECTOR_REGEXP's ID/class/attribute patterns only handled single-character escapes (\.), not hex escapes (\39 ), so they couldn't consume the full escaped identifier.
3. IDs, classes, and attribute names were "unescaped" by just stripping backslashes (/\\/g), which turned \39 7e... into 397e... instead of decoding \39 as the character 9. The cssUnescape() method that handles this correctly already existed — it just wasn't being used for these cases.

Changes

• Added a non-capturing escape sequence alternative to SELECTOR_GROUP_REGEXP so hex/character escapes are consumed before their trailing space can be misread as a combinator
• Updated SELECTOR_REGEXP to match hex escapes (\\[0-9a-fA-F]{1,6}\s?) in ID, class, attribute name, and unquoted attribute value patterns
• Switched ID/class/attribute name handling from the naive backslash strip to cssUnescape()
• Removed the now-unused ESCAPED_CHARACTER_REGEXP

Testing

• Added tests for CSS.escape() with a UUID ID, hex escapes in ID selectors, and hex escapes in class selectors
• All existing querySelector tests still pass

Fixes #2088

[coffeeandwork]

The CI failure is in SyncFetch.test.ts — specifically the cache revalidation test that checks If-Modified-Since behavior with a small max-age. It's a timing issue: when CI is slow, the cache expires before the second request goes out, so the test sees stale headers (max-age=0.0001) instead of the revalidated ones (max-age=1).

This is a known flaky test — #2013 tracks it and there are already PRs (#2014, #2116) trying to fix it. Unrelated to the selector parser changes here. All 157 querySelector tests passed.