docs(release): add 2.9.0 notes (modern-di 2.25 seams adoption)

planning/releases/2.9.0.md

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+# modern-di-fastapi 2.9.0 — modern-di 2.25 integration seams
+
+Maintenance release. **No public API change** — `setup_di`, `FromDI`,
+`build_di_container`, and the context providers keep their signatures and
+behavior. It adopts modern-di 2.25's blessed integration seams and raises the
+floor.
+
+## Internal refactors
+
+- **`container.add_providers(...)` replaces the `providers_registry` reach-in.**
+  `build_di_container` registered `_CONNECTION_PROVIDERS` by poking
+  `container.providers_registry.add_providers(...)` directly; it now calls the
+  public `container.add_providers(...)` seam.
+- **`container.resolve_dependency(...)` replaces the copy-pasted marker
+  dispatch.** `Dependency.__call__` used to branch on
+  `isinstance(self.dependency, providers.AbstractProvider)` to choose between
+  `resolve_provider` and `resolve` itself; that dispatch now lives in modern-di's
+  own `resolve_dependency`, so `FromDI` no longer duplicates logic modern-di
+  maintains.
+
+## Packaging
+
+- **`modern-di` floor raised to `>=2.25,<3`** (was `>=2.21.0,<3`). The seams
+  above (`Container.add_providers`, `Container.resolve_dependency`) were added
+  in modern-di 2.25.0 (modern-python/modern-di#283).
+
+## Downstream
+
+Requires **`modern-di>=2.25`**. There is no API or behavior change, so
+application code that uses `setup_di` / `FromDI` needs no edits — only the
+`modern-di` floor moves.
+
+## Internals
+
+- 100% line coverage maintained; `ruff`, `ty`, and `eof-fixer` clean.
+- `architecture/container-lifecycle.md` and `architecture/dependency-resolution.md`
+  updated to describe the new seams.