[PHP] Limit structured URL rewriting to mapped base components

[adamziel]

BlockMarkupUrlProcessor now changes only the mapped base in each decoded structured URL before passing the result to the existing setter.

For this mapping:

http://old.example/media => https://new.example/assets

this decoded URL:

http://old.example/media/a/./b/../%7e//boat?raw=%2f+%20#Frag%2f

becomes:

https://new.example/assets/a/./b/../%7e//boat?raw=%2f+%20#Frag%2f

The unmatched path, query, fragment, and percent-escape spelling come from the input value. The enclosing serializer still owns HTML entities, CSS quoting, and block JSON escaping. Text-node URL handling is unchanged.

WPURL::replace_base_url() owns the slash-delimited source locator and returns one validated, source-preserving decoded URL. Spellings whose base boundary cannot be located safely are left alone. BlockMarkupUrlProcessor::replace_base_url() only selects the text or structured result and calls set_url(). The setter handles source no-ops, and repeated writes to one CSS token are last-write-wins. This removes the replacement loop, CSS materialization and reseeking, and CSS URL index state from BlockMarkupUrlProcessor.

The rewrite cache stores the selected mapping rather than a rendered URL, so every structured occurrence is converted from its own decoded spelling.

Testing

The focused suites pass:

WPURLTest: 26 tests, 63 assertions
BlockMarkupUrlProcessorTest: 60 tests, 158 assertions
CSSProcessorTest: 264 tests, 365 assertions
CSSURLProcessorTest: 94 tests, 205 assertions
RewriteUrlsTest: 18 tests, 18 assertions

The complete Data Liberation suite ran 2,392 tests. Its only local failures were two remote-fixture DNS errors and the dependent frontloading assertion while network access was disabled. Changed files pass PHP 7.2 compatibility checks and git diff --check; PHPCS reports only the same two pre-existing CSS alignment warnings.