chore: add UPGRADING.md with changelog-driven upgrade notes

CHANGELOG.md

@@ -15,6 +15,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Added
 
+- `UPGRADING.md` — standing upgrade guide; documents that all 0.x releases are additive with no breaking configuration changes
 - Engine-scoped error pages — 404 (`ActiveRecord::RecordNotFound`) and 500 (unhandled `StandardError` in production) now render within the dashboard chrome instead of falling through to the host app's error pages; in development `consider_all_requests_local` keeps the standard Rails debug page
 - Covering indexes added to dummy app schema — `solid_queue_jobs (finished_at, created_at)` for the slow-job scan; `(queue_name, created_at)` on `solid_queue_scheduled_executions` and `solid_queue_blocked_executions` (both previously lacked a queue-name index)
 - Install generator — `rails generate solid_stack_web:install` creates `config/initializers/solid_stack_web.rb` with every config option documented inline and injects the mount line into `config/routes.rb`

ROADMAP.md

@@ -11,7 +11,8 @@ The path to v1.0.0 is staged: first achieve feature parity with `solid_queue_das
 > _Make it easy to adopt and easy to contribute to._
 
 ### Remaining
-- **Changelog-driven upgrade notes** — `UPGRADING.md` for any breaking configuration changes
+
+_All items complete._
 
 ---
 

UPGRADING.md

@@ -0,0 +1,11 @@
+# Upgrading SolidStackWeb
+
+This document covers breaking changes between versions and the steps required to upgrade. Only versions that introduce breaking changes are listed — if your current version is not mentioned, upgrading requires no configuration changes beyond adding the new gem version to your `Gemfile`.
+
+For a full list of changes in every release, see [CHANGELOG.md](CHANGELOG.md).
+
+---
+
+## Upgrading to 0.9.0
+
+No breaking changes. All new configuration keys introduced in 0.x releases are optional with documented defaults; existing initializers require no edits.