fix(P4): trim/split 8 overflowing slides + remove header (REQ-261)

Split slide 3 (Objectives + Anti-Goals) into Slide 3 (Objectives)
+ Slide 4 (Anti-Goals). Split slide 8 (Attestation Matrix) into
Slide 9 (QA concerns, 3 rows) + Slide 10 (Prod/DR concerns, 7 rows).
Main slide count 18 -> 20.

Trimmed: slide 7 (Pipeline) reduced to 3 bullets (4th covered by
diagram). slide 11 (Telemetry) reduced to 3 bullets. slide 14
(Deferred) merged 3 Live-AWS rows into 1 (8 -> 6 rows). slide 17
(Quarter-by-Quarter) dropped Grounding column (5 -> 4 cols). Global
table cell padding reduced (6px 10px -> 4px 8px) so 8-13 row tables
fit.

Removed header: from frontmatter (keep footer: + paginate only).
The full 51-char deck title in BOTH header and footer was redundant
chrome eating ~35px on every slide.

Updated test_marp_deck_slide_count (18 -> 20 main + 1 appendix).
Updated README slide-count convention (18 -> 20).

---ci---
project: acdl
phase: 4
milestone: v1.22
status: execute
phase_role: execution
---/ci---

docs/presentations/README.md

@@ -126,10 +126,11 @@ docs/presentations/
 
 ### Appendix structure
 
-Each Marp deck has **18 main slides + 1 appendix slide**. The main 18 are the
-presentation; the appendix is for Q&A backup.
+Each Marp deck has **20 main slides + 1 appendix slide**. The main 20 are the
+presentation; the appendix is for Q&A backup. (v1.22 split slides 3 and 8
+to relieve overflow, increasing the count from 18 to 20.)
 
-- **Main slides** (1-18): the story arc — Problem → Solution → Proof →
+- **Main slides** (1-20): the story arc — Problem → Solution → Proof →
   Roadmap + Ask. These are what the audience sees during the talk.
 - **Appendix slide** (A1): the Metrics Glossary — detail-heavy reference for
   Q&A.