docs(release): add 2.21.0 notes (ContextProvider.context_type)

planning/releases/2.21.0.md

@@ -0,0 +1,21 @@
+# modern-di 2.21.0 — `ContextProvider.context_type`
+
+Purely additive. One newly-public attribute; the contract of existing code is unchanged.
+
+## Feature
+
+- **`ContextProvider.context_type`** is now a public attribute — the type the provider supplies and the key its value is set under in `context`. It was stored privately (`_context_type`) with no accessor.
+
+Implemented as a plain public slot, matching its sibling `bound_type` (both derive from the same constructor argument) and the base provider's public `scope` / `bound_type`. Properties in the providers are reserved for *derived* values (`display_name`); `context_type` is stored config, so it is a plain attribute.
+
+## Why
+
+Framework integrations build a connection → scope → context-key mapping. Without a public `context_type` they re-state, in an `isinstance` ladder, the very type they already passed into the provider — so the connection-kind knowledge is split across two places. Exposing `context_type` lets an integration drive that dispatch off the provider objects themselves, single-sourcing the mapping.
+
+## Downstream
+
+No action required. Integrations that want to single-source their connection-kind dispatch (the `modern-di-fastapi` refactor, and the same pattern in `modern-di-litestar` / `modern-di-faststream` / `modern-di-typer`) can read `provider.context_type` and bump their floor to `modern-di>=2.21.0`. Nothing breaks for consumers that don't.
+
+## Internals
+
+- 100% line coverage maintained across Python 3.10–3.14; `ruff` and `ty` clean.