feat(studio): render with preview variables + template handoff + docs

Sixth PR of the template-variables Studio stack — closing the loop from
preview to render to developer handoff.

- renders started from the Renders tab now carry the active preview
  variable overrides (StartRenderOptions.variables → POST /render →
  RenderConfig.variables), so "render" produces exactly what the user is
  previewing.
- Variables panel "Use this template" footer: copy the effective values
  (defaults merged with overrides) as JSON, or as a ready-to-run
  `npx hyperframes render <comp> --variables '<json>'` command.
- gitignore: negate the renders/ output rule for the tracked
  src/components/renders/ source dir — without it, pre-commit's format
  re-stage (`git add {staged_files}`) hard-fails on any change to those
  files.
- docs: the Studio panel docs/concepts/variables.mdx described was
  aspirational — replace with the real Variables-in-Studio section
  (declare/edit, render-truthful preview, render-with-values, handoff,
  usage badges); document the new SDK variable APIs in
  docs/sdk/reference/composition.mdx (declaration ops, read APIs,
  setPreviewVariables).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

packages/studio/src/components/StudioRightPanel.tsx

@@ -16,6 +16,7 @@ import { SlideshowPanel } from "./panels/SlideshowPanel";
 import type { SceneInfo } from "./panels/SlideshowPanel";
 import { VariablesPanel } from "./panels/VariablesPanel";
 import { PanelTabButton } from "./PanelTabButton";
+import { usePreviewVariablesStore } from "../hooks/previewVariablesStore";
 import type { RenderJob } from "./renders/useRenderQueue";
 import type { BlockParam } from "@hyperframes/core/registry";
 import type { IframeWindow } from "../player/lib/playbackTypes";
@@ -421,6 +422,9 @@ export function StudioRightPanel({
           format,
           resolution,
           composition,
+          // Render what the user is previewing: active variable overrides
+          // from the Variables panel ride along (undefined = defaults).
+          variables: usePreviewVariablesStore.getState().values ?? undefined,
         });
       }}
       compositionDimensions={compositionDimensions}