chore(docs): replace d2 diagram pipeline with mermaid

Remove docs/_diagrams/ and d2 build step from Makefile. The HTML
template already initialises mermaid.js; diagrams are now inline
code fences rendered client-side.

docs/_diagrams/architecture.d2

@@ -1,65 +0,0 @@
-grid-rows: 2
-grid-gap: 40
-
-test_run: {
-  label: "Test run"
-  grid-rows: 2
-  grid-gap: 100
-
-  go_binary: sanderling (Go) {
-    direction: right
-
-    bundler: Bundler\nesbuild
-    verifier: Verifier\ngoja + LTL
-    runner: Runner
-    driver: Driver
-    trace: Trace writer\nJSONL + PNG
-
-    bundler -> verifier
-    verifier <-> runner
-    runner -> driver
-    runner -> trace
-  }
-
-  platform: {
-    label: ""
-    direction: right
-    style.stroke-width: 0
-    style.fill: transparent
-
-    device: Emulator / device {
-      sdk: sanderling-sdk\npause / state\ncoverage / logs
-    }
-
-    sidecar: Maestro sidecar (JVM) {
-      maestro: maestro-client
-    }
-
-    sidecar.maestro -> device.sdk: UIAutomator
-  }
-
-  go_binary.driver -> platform.sidecar.maestro: gRPC
-  go_binary.runner -> platform.device.sdk: Unix socket
-}
-
-viewer: {
-  label: "Inspect"
-  grid-columns: 4
-  grid-gap: 40
-
-  left_pad: "" {
-    style.stroke-width: 0
-    style.fill: transparent
-    style.opacity: 0
-  }
-  web: Web UI (React)
-  inspect: sanderling inspect\nHTTP + SSE
-  runs: runs/ {
-    shape: cylinder
-  }
-
-  runs -> inspect
-  inspect -> web
-}
-
-test_run.go_binary.trace -> viewer.runs