Add offline mosaic EXIF tagging (stitch --write-exif, tag_mosaic_exif CLI)

- spruce.exif: tag_mosaic_jpeg_for_scan_dir, resolve_machine_label_for_scan_dir; ProcessingSoftware for tile-stitched mosaics
- spruce.settings: load_config(require_credentials=False) for config without login
- scripts/tag_mosaic_exif.py and tests; stitch script --write-exif path

tests/test_settings.py

@@ -95,6 +95,16 @@ def test_load_config_missing_password_exits(tmp_path):
         load_config(str(path))
 
 
+def test_load_config_optional_credentials(tmp_path):
+    path = tmp_path / "config.yaml"
+    path.write_text(
+        yaml.dump({"machine_metadata": {"A [B]": {"plot_number": 2}}})
+    )
+    cfg = load_config(str(path), require_credentials=False)
+    assert cfg["machine_metadata"]["A [B]"]["plot_number"] == 2
+    assert cfg["write_exif"] is True
+
+
 # ---------------------------------------------------------------------------
 # CSV schemas (failure columns)
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