Add canonical collection_picker component

[ty13r]

Summary

Reworks the original app-specific bundle_rail draft into a canonical collection_picker component.

The rail remains the existing canonical right_rail; collection_picker is a reusable form_control_and_composer widget that can be placed inside that rail or any other host-owned surface. Canonical attrs stay generic and renderer-neutral: no bundle-specific fields, Ariston names, route helpers, LiveView event names, or phx-* fields are stored in IUR.

Architecture Decisions

• collection_picker is now first-class in Unified IUR, Unified UI DSL/catalog/lowering, Live UI rendering, Ash UI canonical conversion, and resource admission.
• The old bundle_rail concern is handled as composition: host apps use right_rail plus collection_picker, rather than adding a second canonical rail primitive.
• Drag/drop remains semantic interaction intent at the canonical layer; renderer-specific phx-* attributes are created only at the Live UI boundary.
• Items, filters, and suggestions use generic id/label maps. App-specific item cards remain app-owned composition, not canonical picker payload.
• density is a semantic prop on the picker, not a hard-coded width or theme decision.

What Changed

• Added UnifiedIUR.Widgets.Components.collection_picker/1 and validation for the canonical shape.
• Added Unified UI DSL admission, verifier rules, compiler lowering, and catalog coverage.
• Added a native Live UI CollectionPicker component and renderer mapping from canonical interactions to renderer attrs.
• Added Ash UI adapter and fallback rendering support.
• Updated widget docs/specs/examples and canonical component contracts.
• Added missing Phase 32/33 conformance traceability discovered while running the full suite.

Verification

• mix test - 1064 tests, 0 failures
• mix ash_ui.examples.validate
• bash ./scripts/validate_specs_governance.sh
• git diff --check
• Explicit mix format --check-formatted ... over touched Elixir files
• mix spec.next / mix spec.check --base HEAD attempted; these Mix tasks are not wired in this checkout