Add filter for post preload URLs

[cbratschi]

Summary

• add a narrow wpsc_preload_post_url filter around the permalink selected by the preload worker
• pass the post ID so multilingual and other compatibility layers can resolve the correct URL
• preserve the existing get_permalink() result when no filter is registered
• add WordPress integration coverage for both the default and filtered URL

Context

The preload worker queries published post IDs directly and resolves every URL in the current request language. Multilingual integrations therefore cannot select the language associated with each post without patching the preload worker or changing global language state.

This generic hook adds the missing extension point without adding a dependency on WPML or changing existing behavior. A compatibility layer can inspect the supplied post ID and return the language-specific permalink.

Related: #876

Testing

• composer lint
• php vendor/bin/phpunit -c phpunit.9.xml.dist --colors=always — 84 tests, 111 assertions
• make test-integration — 55 tests, 151 assertions