P0: Prioritize usable video placement in layout preview

[e35dev-loop-maintainer]

Build target

Shift maintainer acceptance toward PRs that make the preview canvas functionally support assigning uploaded/synced videos into layout slots, preserving those placements, and showing the result clearly. Deprioritize thin navigation/link PRs until the core placement workflow is usable.

Maintainer guidance

This operator-approved direction updates how the maintainer should evaluate future PRs for Podcast Design Canvas.

Accept PRs that:

• Accept PRs that let users place or map actual video sources into visible layout slots from the preview or layout-first flow.
• Accept PRs that preserve placed videos when switching compatible layouts and clearly indicate missing, duplicate, or invalid slot assignments.
• Accept PRs with tests or preview evidence covering the full path: choose layout, add/map videos, see videos in slots, continue without losing placement.

Reject or close PRs that:

• Reject or close one-line link/navigation PRs under P0: Example podcast layout canvas with drag-in video slots #1005 when they do not improve the user's ability to put videos into layouts.
• Reject PRs that only expose internal canvas examples without connecting them to real uploaded or synced media.
• Reject changes that make placement state browser-only if the backend-owned loop/state model is the intended source of truth.

Success signal

A user can open the preview, assign distinct videos to layout slots, see them rendered in the selected layout, switch compatible layouts without losing placement, and continue only when the layout has valid media assignments.