Add a Deployment Scenarios page with per-scenario diagrams.

Four mini-diagrams — templates bundled with the app, resolved by a BFF,
shipped with the page, and fetched remotely — each with prose linking to
the specification sections involved. The page makes Section 6.3 visual:
all four deployments are conforming, the transports pair freely with any
of them, and template sources sit outside the protocol. Diagrams are
generated from the committed d2 sources in light and dark variants and
follow the site palette via Material's only-light/only-dark images.

Author: jnbdz

docs/assets/diagrams/scenario-bff-dark.svg

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+# Scenario: a BFF resolves and renders server-side (spec §7.5, §6.3).
+direction: down
+style.fill: transparent
+
+browser: Browser
+bff: BFF {
+  client: VDP client
+  store: Local template store\n(or cached fetches) {
+    style.stroke-dash: 4
+  }
+  client -> store: {
+    style.stroke-dash: 4
+  }
+}
+api: API
+
+browser -> bff.client: GET /dashboard
+bff.client -> api: data +\nview descriptor
+bff.client -> browser: rendered HTML —\nno VDP in the browser

docs/assets/diagrams/scenario-bff-light.svg

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+# Scenario: templates ship inside the app package (spec §6.3).
+direction: down
+style.fill: transparent
+
+api: API
+app: Mobile / desktop app {
+  client: VDP client
+  bundle: Templates shipped\nin the app package {
+    style.stroke-dash: 4
+  }
+  client -> bundle: looks up by URL —\nno fetch {
+    style.stroke-dash: 4
+  }
+}
+
+api -> app.client: data +\nview descriptor

docs/assets/diagrams/scenario-bff.d2

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+# Scenario: templates ship with the page itself (spec §6.3).
+direction: down
+style.fill: transparent
+
+api: API
+page: Browser page {
+  script: VDP client script
+  reg: <template> elements\ndelivered with the HTML {
+    style.stroke-dash: 4
+  }
+  script -> reg: matches URLs\nagainst the registry {
+    style.stroke-dash: 4
+  }
+}
+
+api -> page.script: data +\nview descriptor