Move public assets to stable paths

docs/shared-assets.md

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+# Files shared between development machines
+
+Commit these website assets together with the source code:
+
+- `public/images/example-map.webp` — homepage illustration and map-card fallback.
+- `public/tools/mod/minegeo-fabric-1.21.11.jar` — Fabric mod download.
+- `public/tools/plugin/minegeo-plugin-1.21.11.jar` — server plugin download.
+
+Next.js serves these directly at `/images/example-map.webp`,
+`/tools/mod/minegeo-fabric-1.21.11.jar`, and
+`/tools/plugin/minegeo-plugin-1.21.11.jar`. They are deliberately tracked, so a
+fresh checkout on macOS has the homepage image and both download buttons without
+building Minecraft tools or running the renderer proof.
+
+After changing the Java source, run `npm run java:build` with Java 21 installed.
+The build copies the new JARs into `public/tools/`; commit those updated files too.
+Gradle caches, intermediate build output, and generated test panoramas remain
+ignored. The JARs contain Java bytecode and work across Windows, macOS, and Linux.
+
+On the Mac, pull the commit, run `npm install`, and restart the development
+server. For a production server, run `npm run build` and then `npm start`.
+Keep that machine's Supabase credentials in its own `.env.local`.