chore: start 0.2.4 development cycle

Open the next development cycle after the v0.2.3 release:
- bump version 0.2.3 -> 0.2.4 (Cargo.toml, Cargo.lock)
- add empty upgrade script sql/pg_durable--0.2.3--0.2.4.sql (no schema
  changes yet)
- add a '## [0.2.4] - Unreleased' placeholder to CHANGELOG.md

Part of #243.

Author: pinodeca

CHANGELOG.md

@@ -4,6 +4,8 @@ All notable changes to this project are documented in this file. The format is b
 
 Pre-1.0 note: while `pg_durable` is in major version `0`, minor releases may include breaking changes.
 
+## [0.2.4] - Unreleased
+
 ## [0.2.3] - 2026-06-17
 
 Provider-line note: v0.2.3 stays in the `duroxide-pg` provider compatibility line started in v0.2.2, so the upgrade source is v0.2.2 (`sql/pg_durable--0.2.2--0.2.3.sql`).

Cargo.lock

@@ -1,6 +1,6 @@
 [package]
 name = "pg_durable"
-version = "0.2.3"
+version = "0.2.4"
 edition = "2021"
 license = "PostgreSQL"
 repository = "https://github.com/microsoft/pg_durable"

Cargo.toml

@@ -0,0 +1,9 @@
+-- Copyright (c) Microsoft Corporation.
+-- Licensed under the PostgreSQL License.
+
+-- pg_durable upgrade: 0.2.3 → 0.2.4
+--
+-- No schema changes yet in this release.
+-- Add DDL here for any df-schema changes that land in 0.2.4. See
+-- docs/upgrade-testing.md for the upgrade-script and backward-compatibility
+-- requirements (Scenario A / B1 / B2).