docs: clarify rspack wrapper peer deps (#1193)

## Summary
- Clarify that importing through `shakapacker-rspack` lets pnpm/Yarn
users drop only the direct `shakapacker` dependency
- Keep the required Rspack peers and `rspack-manifest-plugin` documented
as explicit dependencies for strict package managers
- Follow-up to #1185 and
#1185 (comment)

## Validation
- `yarn prettier --check docs/common-upgrades.md
packages/shakapacker-rspack/README.md`
- `git diff --check`
- Ruby local markdown link existence check for touched docs
- `yarn test --runInBand`
- `.agents/bin/validate` progressed through RuboCop, ESLint, and gem
specs (`1197 examples, 0 failures, 6 pending`), then stopped in local
dummy browser specs because ChromeDriver supports Chrome 146 while
installed Chrome is 149. Hosted CI should verify that browser lane with
its runner browser/driver pair.

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Documentation**
* Clarified Rspack upgrade guidance for apps using supplemental
packages.
* Updated instructions to explain when the direct `shakapacker`
dependency can be removed after switching config imports.
* Added clearer dependency requirements so the needed Rspack packages
and manifest plugin remain explicit.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Author: justin808

docs/common-upgrades.md

@@ -104,7 +104,7 @@ Shakapacker 10.1 introduces two optional npm packages — `shakapacker-webpack`
 
 **This is opt-in.** Apps that don't change anything keep working on 10.1 exactly as they did on 10.0.
 
-**Rspack apps** on npm 7+ can replace `shakapacker` + `@rspack/core` + `@rspack/cli` + `@rspack/dev-server` + `rspack-manifest-plugin` with a single `shakapacker-rspack` dev dependency. npm <7, Yarn Classic, pnpm, and Yarn PnP apps should list `shakapacker-rspack` plus those direct imports explicitly unless their config imports the supplemental wrapper directly.
+**Rspack apps** on npm 7+ can replace `shakapacker` + `@rspack/core` + `@rspack/cli` + `@rspack/dev-server` + `rspack-manifest-plugin` with a single `shakapacker-rspack` dev dependency. npm <7, Yarn Classic, pnpm, and Yarn PnP apps should list `shakapacker-rspack`, `shakapacker`, and the required Rspack peers explicitly. If the app changes its config imports from `shakapacker/rspack` to `shakapacker-rspack`, it can drop only the direct `shakapacker` dependency; `@rspack/core`, `@rspack/cli`, `@rspack/dev-server`, and `rspack-manifest-plugin` still need to stay explicit.
 
 **Webpack apps** on npm 7+ can replace `shakapacker` + `webpack` + `webpack-cli` + `webpack-assets-manifest` with a single `shakapacker-webpack` dev dependency. npm <7, Yarn Classic, pnpm, and Yarn PnP apps should list direct imports explicitly. One caveat: `shakapacker-webpack` pins `webpack-assets-manifest` to `~6.5.1`, so apps still on `webpack-assets-manifest@5.x` need to upgrade to v6 when adopting it.
 

packages/shakapacker-rspack/README.md

@@ -55,4 +55,4 @@ If your app already runs Shakapacker on Rspack, you can drop the managed-build d
 }
 ```
 
-Optional peers (`@rspack/plugin-react-refresh`, `css-loader`, `sass`, `sass-loader`) stay only if your app uses those features. Run `yarn install` (or the npm/pnpm equivalent) and the lockfile collapses to the managed stack. npm 7+ can auto-install the required peers; pnpm and Yarn users should keep `shakapacker`, `@rspack/core`, `@rspack/cli`, `@rspack/dev-server`, and `rspack-manifest-plugin` as explicit `devDependencies` unless their config imports the wrapper package directly.
+Optional peers (`@rspack/plugin-react-refresh`, `css-loader`, `sass`, `sass-loader`) stay only if your app uses those features. Run `yarn install` (or the npm/pnpm equivalent) and the lockfile collapses to the managed stack. npm 7+ can auto-install the required peers; pnpm and Yarn users should keep `shakapacker`, `@rspack/core`, `@rspack/cli`, `@rspack/dev-server`, and `rspack-manifest-plugin` as explicit `devDependencies`. If the app changes its config imports from `shakapacker/rspack` to `shakapacker-rspack`, it can drop only the direct `shakapacker` dependency; the Rspack packages and manifest plugin still need to stay explicit.