Wolfram keeps a video's poster frame

Clients could only show a video post as a one-line note. Wolfram now reads the poster frame, its alt text and its declared aspect ratio from an app.bsky.embed.video#view, in both wf_post_display and wf_post_embed.

Changes

• video_thumb, video_alt, video_width, video_height on both structs. The playlist is deliberately not kept: no client here plays video. A ratio is kept only when both sides are present, so half a ratio never reaches a caller that divides by it.
• The recordWithMedia case reads the same fields from its media half.
• Tests: test_embed_video in test_post_display.c, and a video case in test_post_view_typed.c.

Verification

• post_display and post_view ctests pass on the macOS host build. Other platforms were not run locally; the release's CI covers them.

Shipped in v0.37.0 (wolfram#184, released via #185). Cobalt and Indigo consume it: see the video poster entry.