Size the indicator tray from block implicit sizes

Deriving the tray's implicit size from childrenRect fed layout results
back into the bindings that produced them, tripping implicitWidth
binding loop warnings. Compute it from the active and inactive blocks'
own implicit sizes instead, and center the loaded blocks rather than
anchor-filling containers that are sized by their content. The contract
test now instantiates the tray to check the collapse/expand cycle and
fails on any implicitWidth binding loop in the log.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

test/shell.d/indicator-contract-test.sh

@@ -63,4 +63,9 @@ if ! jq -e '.ok == true' "$result" >/dev/null; then
   fail "QML indicator contract checks pass"
 fi
 
+if rg -q 'Indicators.qml.*Binding loop detected for property "implicitWidth"' "$log"; then
+  sed -n '1,200p' "$log" >&2
+  fail "indicator tray avoids implicit-width binding loops"
+fi
+
 pass "QML indicator contract checks pass"