docs(plc): add soft-PLC architecture + CODESYS/Yocto lifecycle diagram

A CODESYS controller is not a monolithic appliance — it's a SoC + Linux +
software runtime + control app, built along two independent tracks (the Yocto
device platform by the OEM, the IEC 61131-3 control application by the machine
builder). Add an "Anatomy" section to the PLC guide: the classic-vs-soft-PLC
table, a Mermaid diagram of the two development tracks converging on the running
device with the artifact→scan mapping, and a callout on where Yocto fits (the
firmware layer, its own artifact). Explains why PlcSps is a composite target.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/guide/plc.md

@@ -11,6 +11,72 @@ the control application *and* the device it runs on.
 | A device firmware image | Firmware SBOM / CVE (opt-in) |
 | A reachable endpoint (WebVisu, OPC UA) | DAST / pentest (opt-in) |
 
+## Anatomy: a soft PLC is a SoC + Linux + runtime
+
+A CODESYS controller is **not** a monolithic appliance like a classic Siemens
+S7. It is **PC-based ("soft") control** — commodity silicon running a
+general-purpose Linux, with a **software PLC runtime** as just another process:
+
+| Classic PLC (e.g. Siemens S7) | Soft PLC (CODESYS-on-Yocto, OpenPLC-on-Raspbian) |
+| --- | --- |
+| Proprietary hardware + firmware | Commodity SoC (x86 / ARM) |
+| Proprietary OS | General-purpose Linux (a **Yocto** image, or Raspbian) |
+| Proprietary runtime | Software runtime (**CODESYS Control**, or OpenPLC) |
+| STEP7 / TIA project | IEC 61131-3 control app (ST / LD / FBD / SFC) |
+
+Because of this, the device is built along **two independent tracks**, by
+different people, on different timelines, and shipped separately. It also
+inherits the **entire Linux / IT attack surface on top of** the OT / control
+one — which is exactly why a PLC/SPS target is treated as a **composite**:
+Certifai ingests one artifact per layer and scans each with the right pipeline.
+
+```mermaid
+flowchart TB
+    subgraph TA["Track A · Device platform — built by the hardware OEM / vendor"]
+        direction LR
+        A1["Yocto / OpenEmbedded<br/>BSP + RT kernel"] --> A2["Bake in the CODESYS<br/>Control for Linux runtime"] --> A3["bitbake → device image<br/>.wic / .tar + manifest"]
+    end
+    subgraph TB2["Track B · Control application — built by the machine builder / customer"]
+        direction LR
+        B1["CODESYS IDE<br/>ST / LD / FBD / SFC + WebVisu"] --> B2["Reference CODESYS +<br/>vendor libraries"] --> B3["Compile → download<br/>to device (gateway 11740)"]
+    end
+
+    A3 --> DEV(["Running soft-PLC device<br/>SoC + Linux + runtime + control app<br/>Modbus · OPC UA · EtherNet/IP · WebVisu"])
+    B3 --> DEV
+
+    subgraph CERT["What Certifai scans — one layer per artifact"]
+        direction LR
+        S1["Firmware layer<br/>FirmwareStatic · SBOM · CVE"]
+        S2["Control-logic layer<br/>PLC SAST — ST + FBD/LD"]
+        S3["Control-app SBOM<br/>libraries + runtime → CVE"]
+        S4["Running layer<br/>ICS probe · DAST (WebVisu)"]
+    end
+
+    A3 -. firmware image .-> S1
+    B1 -. PLCopen XML / ST via git .-> S2
+    B2 -. projectarchive (zip) .-> S3
+    DEV -. live URL / provisioned .-> S4
+
+    classDef yocto fill:#fde68a,stroke:#b45309,color:#111
+    classDef codesys fill:#bfdbfe,stroke:#1d4ed8,color:#111
+    classDef dev fill:#e9d5ff,stroke:#7e22ce,color:#111
+    classDef cert fill:#bbf7d0,stroke:#15803d,color:#111
+    class A1,A2,A3 yocto
+    class B1,B2,B3 codesys
+    class DEV dev
+    class S1,S2,S3,S4 cert
+```
+
+::: tip Where Yocto fits
+Yocto is **Track A** — the *build system* for the device platform. It produces
+the Linux image and bakes in the CODESYS runtime, so it is the **firmware
+layer**, entirely separate from the control application. Hand it to Certifai as
+its own **firmware image** artifact (scanned by the firmware pipeline, not the
+PLC pipeline). The device OS need not be Yocto — Raspbian/Debian/Buildroot, or
+even an RTOS / bare-metal, are all possible — but Yocto is the common,
+product-grade industrial choice.
+:::
+
 ## Two ways to deliver the project
 
 You can either **upload** the project when onboarding, or point Certifai at a