docs: define core Mighty terminology

CONTEXT.md

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+# Mighty
+
+Mighty lets a backend of any language render Astro pages on demand, the way Inertia.js lets a backend drive a JavaScript SPA. This glossary fixes the words the packages, docs, and issues use for that.
+
+## Language
+
+**Adapter**:
+The integration between one backend framework (Hono, Laravel) and Mighty. An adapter turns a backend request into a render request and returns the result as a backend response.
+_Avoid_: Integration, plugin, driver
+
+**Render request**:
+The unit of work an adapter hands to Mighty: which Astro page component to render, with which props and shared context, and whether to render it as a partial.
+_Avoid_: Render call, page request
+
+**Shared context**:
+Per-request data the backend exposes to every Astro component in a render, outside of props. Auth user, session flashes, and CSRF tokens are typical.
+_Avoid_: Globals, shared props, locals
+
+**Sidecar**:
+A separate Node process that serves render requests over a local transport for adapters whose backend cannot run Astro in-process.
+_Avoid_: Render server, bridge, daemon