Fixes for :scope() and :has() parsing and some error paths.

[wRAR]

1. An invalid - "combinator" inside :has() raises KeyError, not a clean error
2. X|ident silently discards everything parsed before the |; |* is rejected
3. The :scope start-of-selector rule mis-fires in both directions
4. *:nth-last-of-type() error message names the wrong pseudo-class
5. :has() re-tokenizes its argument through a string, accepting strings and breaking escapes
6. :is()/:where() have a misleading error for :not() arguments
7. The :scope rejection message mislabels it as an "immediate child pseudo-element"
8. :nth-child() series parsing is case-sensitive where CSS is not
9. Trailing whitespace inside a :has() argument is a syntax error

Found and fixed by an LLM, all test cases validated manually.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.67%. Comparing base (743c6e5) to head (12467f0).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #168      +/-   ##
==========================================
+ Coverage   96.61%   96.67%   +0.06%     
==========================================
  Files           3        3              
  Lines         885      901      +16     
  Branches      136      142       +6     
==========================================
+ Hits          855      871      +16     
  Misses         14       14              
  Partials       16       16              

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.