* feat(channels): batch 3 producers drop media placeholder bodies
Media-placeholder program batch 3: iMessage, Signal, and WhatsApp stop
minting <media:kind> placeholder bodies. Primary bodies carry captions
only with one aligned structured fact per native attachment (type-only
for missing/rejected/oversized). Echo, quote, and pending-history
records gain structured media facts so dedupe and previews work without
rendered text. Signal media-only outbound sends drop the placeholder
body entirely (signal-cli accepts attachment-only sends; recipients now
see just the media). WhatsApp audio preflight de-stringed; qa-lab
sticker recognizer moved to structured facts; WhatsApp gif maps to
video (presentation mode, no behavioral consumer).
* test(whatsapp): keep media classifier private
