feat: add interactive FC01 heap4 card

web/figures/a2-structure.puml

@@ -0,0 +1,48 @@
+@startuml
+scale 620 height
+top to bottom direction
+skinparam backgroundColor transparent
+skinparam shadowing false
+skinparam defaultFontName Monospace
+skinparam defaultFontSize 10
+skinparam classBorderColor #2c7794
+skinparam classBackgroundColor #edf6fa
+skinparam ArrowColor #8095a1
+title A2 STRUCTURE — in-band metadata and address order
+
+class "01 Block header" as HEADER {
+  next : BlockLink_t*
+  size : size_t
+}
+class "Allocated block" as ALLOC {
+  02 header + 13 byte request
+  aligned total = 32 bytes
+  payload begins after header
+}
+class "03 Free remainder" as REM {
+  own header
+  size = 224 bytes
+}
+
+HEADER *-- ALLOC : precedes payload
+ALLOC -down-> REM : split preserves 256
+
+class "04 Address-ordered free list" as LIST {
+  previous < inserted < current
+}
+class "05 Right neighbour" as RIGHT {
+  block + block.size == current
+}
+class "06 Left neighbour" as LEFT {
+  previous + previous.size == block
+}
+
+REM -down-> LIST : insert by address
+LIST -down-> RIGHT : join first
+RIGHT -down-> LEFT : then join left
+
+note right of LIST
+Ordering makes both physical
+neighbours locally visible.
+end note
+@enduml