fix(engine): centre modal cards within usable area (status-bar + gesture-bar)

apply_safe_area_to_modal_scrims now sets both padding.top (status-bar
height) and padding.bottom (gesture-bar height) on every ModalScrim.
With align_items/justify_content: Center on the scrim, the modal card
lands at the visual midpoint of the visible area between the two system
bars, fixing the slight upward shift that occurred when only the bottom
inset was applied.

Also: mark all rewrite-plan phases (0–3) complete; drop obsolete stash
whose 20 files are already incorporated into master; update CLAUDE.md
§14.3 to document both edges.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

CLAUDE.md

@@ -430,9 +430,11 @@ explicitly replacing the current one (despawn first, then spawn).
 
 ## 14.3 Safe area
 
-Every `ModalScrim` automatically receives `padding.bottom` equal to the
-logical gesture-bar height via `apply_safe_area_to_modal_scrims` in
-`SafeAreaInsetsPlugin`. Do not manually add bottom padding to scrim nodes.
+Every `ModalScrim` automatically receives `padding.top` equal to the logical
+status-bar height and `padding.bottom` equal to the logical gesture-bar height
+via `apply_safe_area_to_modal_scrims` in `SafeAreaInsetsPlugin`. This centres
+the modal card within the usable area between both system bars. Do not manually
+add top or bottom padding to scrim nodes.
 
 ## 14.4 Z-ordering