feat(authoring): allow list_item_multi_column on repeat-bound elements

[ty13r]

Summary

One-line widening: add list_item_multi_column to the @list_widgets MapSet in AshUI.Resources.Validations.Authoring. The canonical renderer already supports this kind on repeat-bound Elements; the authoring validator just hadn't been widened to allow it.

Why

Currently a repeat-bound Element that wants to render multi-column rows must either:

1. Use a custom:* prefix on the type and add a RepeatRowFlattening clause that rewrites the node type to "list_item_multi_column" before render, OR
2. Drop the ui_bindings :list block (which then trips Info.destination_list_binding? at runtime)

Neither is ideal. The renderer has full list_item_multi_column coverage in LiveUIAdapter.generate_heex/2; the validator just rejects the same type declaration that the renderer is ready to handle.

ariston-ui downstream cleanup once this lands

lib/ariston_ui_web/iur/repeat_row_flattening.ex has 3 flatten clauses that exist solely to bridge this gap: flatten_map_repo_row/1, flatten_map_activity_row/1, flatten_map_system_event/1. All three can be removed and the corresponding Elements declared as type "list_item_multi_column" directly. Documented in ariston-ui docs/operator-v2-phase-2-deferred-findings.md (substrate items section, item #2).

Test plan

• mix format --check-formatted passes
• mix compile --warnings-as-errors passes
• mix test test/ash_ui/authoring test/ash_ui/phase_31_canonical_conversion_test.exs test/ash_ui/phase_31_package_boundary_test.exs -> 15 tests, 0 failures

Tier classification

Tier 1 mechanical fix per the substrate-gap co-maintain framing — single-line allowlist widening, no behavioral change to existing callers, no API design choice. Pinging @pcharbon70 for approval; happy to merge once blessed.

🤖 Generated with Claude Code

[ty13r]

Cross-linking the three operator_v2 substrate-gap items (per project-ariston Update 8):

• This PR (feat(authoring): allow list_item_multi_column on repeat-bound elements #142) — Gap 2: @list_widgets allowlist (Tier 1 mechanical)
• feat(rendering): add generate_heex clauses for 5 shell-primitive kinds #144 — Gap 1: generate_heex clauses for 5 shell-primitive kinds (Tier 2 DRAFT)
• Authority.create/2: support explicit :name opt for screen lookup #143 — Gap 3: Authority.create/2 name vs lookup mismatch (Tier 2/3 issue, direction-pick)