Confirm the conformed channel_slug value set against each source's real channel values #2538

Description/Context

dim_feedback_channel is the one facet that survived the ≥2-sources conformance rule — everything else Zendesk-shaped went into the source_metadata variant. That makes it the only attribute available for cross-source comparison of how feedback arrived, so a mapping gap silently makes it useless.

Proposed value set: email | web_form | in_product_widget | chat | forum_post | assessment | api, plus is_solicited — did we ask for this feedback, or did the user volunteer it. Those have different selection biases and should rarely be pooled without saying so.

Plan/Design

Confirm the set against the actual distinct values of Zendesk ticket_source_channel / comment_source_channel first, then sanity-check the forum/tutor/ORA/plugin mappings before seeding.

Requirement: an unmapped source value must fail loudly in int__feedback__zendesk rather than bucket to a catch-all. An unnoticed mapping gap is exactly the failure this dimension cannot tolerate.

Spec: feedback_dimensional_model.md §4d