embed and post-author response parity
- structuredContent no longer carries images[].aspectRatio, video.aspectRatio, or external.thumb — neither was ever rendered in content[], and no tool's advertised schema named them, so no client validating against tools/list breaks (#24)
- content[] gains a quoted post's CID beside its AT-URI and each label's src/cts beside its value, closing the gap the other direction (#24)
- BREAKING: bsky_get_post_thread's post author narrows from the full profile view to four fields (did, handle, displayName?, avatar?) — its node schema is passthrough(), so this is a real change to structuredContent; bsky_search_posts and bsky_get_author_feed are unaffected, already closed to that shape (#24)