feat: add the TechLog asset multipart upload transport

Wires the whole Asset capability into the running application: the
multipart upload transport (the contract runtime can only express JSON
bodies), a single composition-root-owned CSRF provider shared between
the platform's credential collaborator (18 JSON operations) and the
upload transport (1 multipart operation), and Studio/StudioShell
exposure of the Asset gateway alongside the existing document gateway.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/features/tech-log/presentation/studio/studio-provider.tsx

@@ -7,6 +7,7 @@ import {
 } from "react";
 
 import { isStudioGatewayError } from "../../application/ports/studio-gateway-error.ts";
+import type { StudioAssetGateway } from "../../application/ports/studio-asset-gateway.ts";
 import type { StudioGateway } from "../../application/ports/studio-gateway.ts";
 import type { ResolvePublishedLabel } from "../../domain/public-render-content.ts";
 import type {
@@ -25,6 +26,9 @@ import { useBeforeUnload } from "./components/use-before-unload.ts";
 type StudioProviderProps = Readonly<{
   children: ReactNode;
   createGateway: () => StudioGateway;
+  // Optional so test harnesses that only exercise the document gateway keep
+  // working unchanged. `StudioShell` always supplies one in the running app.
+  createAssetGateway?: () => StudioAssetGateway;
   resolvePublishedLabel?: ResolvePublishedLabel;
   now?: () => Date;
   navigate?: (href: string) => void;
@@ -48,11 +52,18 @@ const missingPublishedLabel: ResolvePublishedLabel = () => undefined;
 export function StudioProvider({
   children,
   createGateway,
+  createAssetGateway,
   resolvePublishedLabel = missingPublishedLabel,
   now = () => new Date("2026-08-14T01:00:00.000Z"),
   navigate = defaultNavigate,
 }: StudioProviderProps) {
   const [gateway] = useState<StudioGateway>(() => createGateway());
+  // Same lazy, called-once-per-mount pattern as `gateway`. Both are keyed to
+  // the same provider generation in `StudioShell`, so a persisted `pageshow`
+  // remount recreates them together — never one without the other.
+  const [assetGateway] = useState<StudioAssetGateway | null>(
+    () => createAssetGateway?.() ?? null,
+  );
   const [editor, setEditor] = useState<StudioEditorState | null>(null);
   const [pendingHref, setPendingHref] = useState<string | null>(null);
   const [requestAnnouncement, setRequestAnnouncement] = useState("");
@@ -148,6 +159,7 @@ export function StudioProvider({
   const value = useMemo<StudioContextValue>(
     () => ({
       gateway,
+      assetGateway,
       resolvePublishedLabel,
       now,
       editor,
@@ -160,6 +172,7 @@ export function StudioProvider({
       clearEditor,
     }),
     [
+      assetGateway,
       beginEditor,
       clearEditor,
       editor,