docs: release notes for 0.5.0

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: lesnik512

planning/releases/0.5.0.md

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+# db-retry 0.5.0 — Python 3.11 and 3.12 support
+
+The supported Python floor drops from 3.13 to **3.11**. `db-retry` now runs on
+Python 3.11, 3.12, 3.13, and 3.14. **There are no behavior or public-API
+changes** — the five public symbols (`postgres_retry`,
+`build_connection_factory`, `build_db_dsn`, `is_dsn_multihost`, `Transaction`)
+and their signatures are unchanged.
+
+## What changed
+
+- **`requires-python` is now `>=3.11,<4`** (was `>=3.13`), with matching PyPI
+  classifiers for 3.11 and 3.12.
+- **`postgres_retry`'s generics were rewritten off PEP 695.** The `type` alias
+  statements and `def postgres_retry[**P, T]` syntax (3.12+ only) became
+  module-level `typing.ParamSpec`/`TypeVar` with `TypeAlias` aliases. No new
+  dependency — those are stdlib since 3.10. The decorator's behavior and the
+  two call forms (`@postgres_retry` / `@postgres_retry(retries=N)`) are
+  identical.
+
+## Downstream
+
+No action needed beyond the wider version support — no API or behavior change.
+A drop-in upgrade from 0.4.3 for projects already on 3.13+, and now installable
+on 3.11 and 3.12.
+
+## Internals
+
+- The refactor landed test-first with 100% coverage maintained.
+- CI now runs the suite on a Python 3.11/3.12/3.13/3.14 matrix, with each leg
+  pinned to its target interpreter so every version is genuinely exercised.