Fix posts query filtering by post_type and term_slug

[dChiamp]

• term_slug no longer overwrites an explicit post_type param; taxonomy inference only runs when post_type is omitted
• term_slug taxonomy search is now scoped to taxonomies registered for the requested post_type(s), preventing cross-CPT term collisions
• post_type and term_slug both accept comma-separated values; multiple term slugs are matched with an OR tax_query grouped by taxonomy