feat(linkedin-studio): video quality gate (captions + aspect guidance, drop 9:16 mandate)

Resolve the video-advice contradiction on the two surfaces this step owns
(research/03 D1-D3):

- commands/video.md: rename the "3-Second Test" to a "Muted-Autoplay Test"
  (front-load value for ~85% muted viewing; the "three-second hook" is
  cross-platform folklore, not a LinkedIn signal); make captions the
  enforceable spec (SRT or native auto-captions, indexed for search);
  aspect ratio as guidance (4:5/1:1 preferred for broad distribution,
  9:16 opt-in for the vertical video tab, crops to 1:1 on desktop); add
  MP4-default + warn-only MOV/AVI + official upload limits to the
  pre-recording reminder.
- references/linkedin-formats.md: reframe the 9:16 "distribution boost"
  as uncorroborated heuristic; 4:5 "deprioritized" -> 4:5/1:1 preferred;
  drop the "3-second hook"; MP4 safe-default + warn-only MOV/AVI; replace
  "good video rewarded more than ever" with the honest "per-video reach
  declining; documents/carousels out-engage video."

algorithm-signals-reference.md (the canonical magnitude source) was
already reconciled in Phase 0 (line 56 reads "declining ... 4:5/1:1
preferred, captions are the enforceable spec") and carries no boost copy,
so it is intentionally untouched here — and it is not in this step's
manifest.

Verify: grep 'must be 9:16|9:16 (1080|3-second hook' video.md
linkedin-formats.md -> none; 'captions' video.md -> 5; 'deprioritized'
linkedin-formats.md -> none; structure lint 61/61.

KNOWN RESIDUAL (flagged for follow-up, NOT in this step's scope):
- references/linkedin-growth-playbook-2025-2026.md:216 still carries a
  "9:16 ... distribution boost" line. That file is owned by Step 17, but
  Step 17's changes are newsletter-distribution — the video line would
  fall through. Fold this into Step 17 or the review gate.
- references/video-strategy-guide.md:300 still says "the 3-second test
  determines 70% of retention." That file is owned by NO plan step
  (orphaned) — needs a home. Surface at session end for an operator
  decision.

Plan Step 15 (Wave 4 S2). Counts unchanged.
[skip-docs]: tre-doc + version bump deferred to Step 21 per remediation plan.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

plugins/linkedin-studio/references/linkedin-formats.md

@@ -58,9 +58,9 @@ Choosing the right format isn't just about engagement rates—it's about underst
    - **Critical:** 30% minimum completion rate or video gets zero distribution
    - LinkedIn Live: 12-24x engagement vs standard posts
    - 85% watch without sound (captions essential)
-   - **Vertical 9:16 (1080×1920)** now gets distribution boost (3-4x watch duration vs landscape). 4:5 still acceptable but deprioritized
-   - First 3 seconds determine 70% of retention — 3-second hook is critical
-   - Note: Overall video reach down 72% YoY — but good video is rewarded more than ever
+   - **4:5 (1080×1350) or 1:1 (1080×1080) preferred** for broad feed distribution on a desktop-heavy professional audience. 9:16 is delivered mobile-only and crops to 1:1 on desktop; its "distribution boost" is an uncorroborated heuristic — no official 4:5-vs-9:16 engagement study exists. Reserve 9:16 for the opt-in vertical video tab
+   - Front-load value for muted autoplay — ~85% watch without sound. The "three-second hook" is cross-platform folklore, not a LinkedIn-named signal; LinkedIn's only official "3 seconds" is the minimum video length
+   - Note: per-video organic reach is declining year-over-year; documents/carousels currently out-engage video — use video when it adds something text can't
    - Best for: Personal stories, quick insights, behind-the-scenes
    - See "Video Content Deep Dive" section below for comprehensive guidance
 
@@ -533,12 +533,12 @@ Video isn't the silver bullet many creators think it is. Text-based thought lead
 ### Technical Specifications
 
 **Video Format & Resolution:**
-- **Aspect ratio:** Vertical 9:16 (1080x1920) now gets distribution boost in LinkedIn's immersive feed
-  - Vertical 9:16: 1080x1920px (optimal for 2026 — 3-4x watch duration vs landscape, 100% mobile viewport)
-  - Vertical 4:5: 1080x1350px (still acceptable)
-  - Square 1:1: 1080x1080px (deprioritized)
-  - If using 16:9: 1920x1080px minimum (only 25% of mobile viewport)
-- **File format:** MP4 (H.264 codec)
+- **Aspect ratio:** 4:5 / 1:1 preferred for broad feed distribution; 9:16 is a mobile-only opt-in (crops to 1:1 on desktop). Aspect ratio is guidance, not an enforceable gate — captions are the spec to enforce
+  - Vertical 4:5: 1080x1350px (preferred — fills the mobile feed, uncropped on desktop)
+  - Square 1:1: 1080x1080px (preferred — safe on every surface)
+  - Vertical 9:16: 1080x1920px (opt-in, for the vertical video tab; delivered mobile-only and crops to 1:1 on desktop — the official 9:16 numeric target is scoped to ads, not organic)
+  - If using 16:9: 1920x1080px minimum (letterboxed in the feed)
+- **File format:** MP4 (H.264) — the safe default. LinkedIn's own sources conflict on MOV/AVI (member troubleshooting lists them supported; the Pages spec says no longer supported), so treat MOV/AVI as warn-only, not blocking — re-encode to MP4 when in doubt
 - **Maximum file size:** 5GB
 - **Maximum length:** 10 minutes (but aim for 30-60 seconds. 30% completion rate minimum or zero distribution)
 - **Frame rate:** 30fps standard, 60fps for smooth motion
@@ -645,7 +645,7 @@ Before posting any video, verify:
 - [ ] Ends with engagement-focused CTA
 
 **Technical:**
-- [ ] Vertical 9:16 format (1080x1920) for maximum reach in immersive feed
+- [ ] Aspect ratio 4:5 (1080x1350) or 1:1 for broad feed distribution — 9:16 only if targeting the opt-in vertical video tab
 - [ ] Professional captions added
 - [ ] Audio quality clear and consistent
 - [ ] Thumbnail captures attention
@@ -659,7 +659,7 @@ Before posting any video, verify:
 - [ ] Complements overall content strategy
 - [ ] Doesn't include external links
 
-**Bottom Line on Video:** Use strategically when it genuinely adds value beyond text. Prioritize authenticity over production quality. Focus on 60-second videos that deliver concentrated insights. LinkedIn now requires 30% minimum completion rate for any distribution — shorter is safer. Always optimize for mobile-first consumption with vertical 9:16 format, captions, and 3-second hooks.
+**Bottom Line on Video:** Use strategically when it genuinely adds value beyond text. Prioritize authenticity over production quality. Focus on 60-second videos that deliver concentrated insights. LinkedIn now requires 30% minimum completion rate for any distribution — shorter is safer. Always optimize for mobile-first consumption: captions on (the one enforceable spec), 4:5/1:1 aspect for broad distribution, and a front-loaded opening for muted autoplay.
 
 
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