fix: preserve ancestor selectors for literal ampersands

[feritcemkahraman]

Nested selectors containing only a quoted or escaped ampersand lose their ancestor selector. For example, .outer { .parent { [data-label="&"] { color: red } } } resolves to [data-label="&"] instead of .outer .parent [data-label="&"].

Use the existing quote/escape/block-aware splitter to decide whether the selector contains a nesting ampersand. Reuse the split parts when substituting an actual nesting selector. Literal-only ampersands now take the normal descendant path.

Validation:

• Original full node --test: 39 passing tests.
• Three new regressions (quoted attribute, escaped class name, attribute inside :is) fail before the source fix.
• Full node --test after the fix: 42 passing tests, including existing actual-ampersand substitution cases.
• git diff --check passes. No runtime dependencies added.

[romainmenke]

thanks!