docs(badges): update print behavior docs + Zebra test day plan

- MODULE__AE_Events_Badge_Templates: rewrite Print Layout Architecture
  section with accurate cross-browser behavior table and Chrome margin
  guidance
- PROJECT__AE_Events_Badges_Review_Print: add TASK 4.1 print CSS
  centering history; correct Chrome squish root cause explanation
- PROJECT__AE_Events_Zebra_Hardware_Test_Day: new — comprehensive
  pre-test and day-of checklists for ZC10L hardware test ~2026-03-16
- TODO__Agents: add [Badges] Zebra ZC10L Hardware Testing section

documentation/PROJECT__AE_Events_Badges_Review_Print.md

@@ -129,6 +129,28 @@ should be the reference. See Design Intent section above.
   Pronouns field is now an attendee-level field (was trusted-only). Staff section labelled.
 - Debug JSON blocks in v2 badge render hidden behind global edit_mode flag.
 
+**Print page CSS centering work (also 2026-03-12) — ⚠️ Chromium PDF issue pending:**
+Multiple iterations to center the badge on the printed page, working around SvelteKit layout
+hierarchy issues. Current approach: `position: fixed; top: 50%; left: 50%; transform: translate(-50%, -50%)`
+on `.event_badge_wrapper`. Key findings:
+
+- `#ae_main_content` has `overflow: auto` — Firefox (spec-compliant) won't let `display: contents`
+  dissolve it. Workaround: explicit passthrough block (`display: block; overflow: visible; width: 100%`).
+- `app.css` global `overflow: hidden` on `html, body` creates a BFC that collapses body to badge-width.
+  Override with `overflow: visible !important` in print CSS.
+- **Chrome ignores `@page { size }` for Save as PDF** — verified 2026-03-12. Firefox honors it.
+  Chrome uses the system default paper size; Firefox locks to the CSS `@page { size }` value.
+  Neither browser lets you change paper size in "Save to PDF" mode — only when printing to a
+  physical printer. For actual Epson/Zebra printing, the driver controls paper size; unaffected.
+- **Chrome "Default" margins cause the "squish"** — Chrome's Default margin setting inserts
+  URL/date/page-number headers and footers into the printable area. These eat into the space
+  that `position: fixed; top: 50%` references, making the badge off-center or clipped. Fix:
+  set **Margins → None** (or Minimum) in Chrome's print dialog. Content centering verified
+  correct: Chrome A4 + None margins produces badge center = 297.5 pts on a 297.5 pt wide page.
+  Use Firefox for accurate PDF proofing.
+- See `documentation/MODULE__AE_Events_Badge_Templates.md` → "Print Layout Architecture" for full
+  technical details.
+
 ### ✅ TASK 3: Badge Print Controls Panel — COMPLETE (2026-03-02)
 
 **Files created/modified:**