Denormalize trait membership (terms/traits sidecar)

Membership runs through the `crtxt_collection` taxonomy, so every document query adds a `tax_query` JOIN over `term_relationships` and `term_taxonomy`, and resolving a document's trait hits the term tables. We accepted that in #280: the JOIN was ~1-3 ms, well inside the noise, and the boot win mattered more. But it scales with the workspace, so at some point it's worth taking off the hot paths.

**Proposed solution:** keep a denormalized copy of document→trait membership, like the field-value index, so the hot paths skip the JOIN. Prime the term lookups (`term_id_for_trait`, `all_trait_ids`, `find_trait_for_document`) once per request too.

Start by tracing the crtxt_collection taxonomy and the existing field-value index through the document-query hot paths. Compare term_id_for_trait, all_trait_ids, and find_trait_for_document, then define completion as denormalized document-to-trait membership that avoids the JOIN while preserving behavior and primes those lookups once per request.

Written by the indexing model from the issue text.