feat(regression): R2 write backend for native baselines

changelog/732.feature.md

@@ -0,0 +1,5 @@
+Added a Cloudflare R2 object-store backend for diagnostic regression baselines, so `ref test-cases mint` can publish a test case's native outputs to the shared baseline store rather than only a local directory.
+The store endpoint and bucket are configured through the `REF_NATIVE_STORE_*` settings (defaulting to the project's public baseline bucket), and write credentials are supplied via the environment or a named AWS/R2 profile, never the persisted configuration.
+
+Minting is also easier to get right.
+`ref test-cases mint` now checks the store credentials and bucket up front and stops with a clear message when they are misconfigured, the new `ref test-cases check-store` command verifies store connectivity without minting, `ref test-cases mint --dry-run` previews what would be minted, and `ref test-cases fetch` restores a missing local paths file on its own so a fresh checkout no longer needs `--force`.

packages/climate-ref-core/pyproject.toml

@@ -56,6 +56,13 @@ dependencies = [
     "fastprogress==1.0.5"
 ]
 
+[project.optional-dependencies]
+# Credentialed S3-compatible write backend for the native baseline store (Cloudflare R2).
+# Only required for the `ref test-cases mint` upload path; read/replay never need it.
+aws = [
+    "boto3>=1.34",
+]
+
 [dependency-groups]
 dev = [
     "types-requests",

packages/climate-ref-core/src/climate_ref_core/regression/__init__.py

@@ -41,6 +41,7 @@
 from climate_ref_core.regression.store import (
     LocalFilesystemStore,
     NativeStore,
+    NativeStoreUnavailableError,
     PoochReadStore,
     R2WriteStore,
     build_native_store,
@@ -55,6 +56,7 @@
     "Manifest",
     "NativeEntry",
     "NativeStore",
+    "NativeStoreUnavailableError",
     "PoochReadStore",
     "R2WriteStore",
     "Tolerance",