Initial CAN Physical Layer spec

Architecture/diagrams/README.md

@@ -1,6 +1,6 @@
 # Architecture diagram sources
 
-The architecture Markdown embeds these 22 SVG figures. Matching PNG previews
+The architecture Markdown embeds these 23 SVG figures. Matching PNG previews
 are provided for viewers that do not support SVG. Diagrams describe the working
 architecture; component qualification and schematic-release requirements remain
 in the parent documents.
@@ -11,6 +11,7 @@ in the parent documents.
 | --- | --- |
 | Power | [Sources](power-sources.svg), [protected branches](power-branches.svg) |
 | Analogue acquisition | [Passive input](analog-input.svg), [thermistor](thermistor.svg), [ratiometric acquisition](ratiometric.svg) |
+| Vehicle CAN FD | [Trunk, termination and service access](can-topology.svg) |
 | Timing inputs | [Crank/cam interface](trigger-input.svg) |
 | Deadman | [Contacts and diagnostics](deadman-inputs.svg), [permit gates](deadman-permit.svg), [output gates](deadman-outputs.svg) |
 | Ignition | [Power](ignition-power.svg), [commands](ignition-command.svg), [current flag](ignition-flag.svg), [grounding](ignition-ground.svg) |
@@ -20,8 +21,10 @@ in the parent documents.
 
 ## Editing and regeneration
 
-- The 12 block figures have editable `.drawio` sources. Open them in draw.io.
-  `build_blocks.py` recreates their baseline layout and `block-connections.json`.
+- The 13 block figures have editable `.drawio` sources. Open them in draw.io.
+  `build_blocks.py` recreates the original 12 block layouts and `block-connections.json`.
+  The CAN topology is maintained directly in `can-topology.drawio`, with its
+  own `can-topology.connections.json` manifest; the builder does not overwrite it.
   Running it overwrites manual layout edits, so update the generator when those
   edits should be reproducible, or export the edited `.drawio` directly.
 - The 10 circuit/logic figures are generated by `build_circuits.py`, with a