fix(scan): resolve "Reg-" prefixed region shortcodes to full names

PR #3035 made region/language shortcode lookups case-sensitive by keying
REGIONS_BY_SHORTCODE on the uppercase codes, and updated the plain tag
lookups to match against the raw (unlowered) tag. The "Reg-" prefix branch
was missed: it still lowercased the captured shortcode before looking it up,
so it could never hit the now-uppercase-keyed dict. As a result a tag like
"[Reg-J]" was stored as the raw "J" instead of resolving to "Japan".

Match the shortcode with its original case, like the other lookups, so
"[Reg-J]" resolves to "Japan" and "[Reg-U]" to "USA". Non-shortcode values
(e.g. "[Reg-PAL]") are still kept verbatim via the fallback.

Co-Authored-By: Claude <noreply@anthropic.com>

Author: TowyTowy

backend/handler/filesystem/roms_handler.py

@@ -214,8 +214,8 @@ def parse_tags(self, fs_name: str) -> ParsedTags:
             # Region prefix
             region_match = REGION_TAG_REGEX.match(raw_tag)
             if region_match:
-                key = region_match[1].lower()
-                regions.append(REGIONS_BY_SHORTCODE.get(key, region_match[1]))
+                region = region_match[1]
+                regions.append(REGIONS_BY_SHORTCODE.get(region, region))
                 continue
 
             # Revision prefix

backend/tests/handler/filesystem/test_roms_handler.py

@@ -272,6 +272,15 @@ def test_parse_tags_non_version_tags_starting_with_v(self, handler: FSRomsHandle
         assert parsed_tags.version == ""
         assert "versionB" in parsed_tags.other_tags
 
+    def test_parse_tags_reg_prefix_resolves_shortcode(self, handler: FSRomsHandler):
+        """A "Reg-" prefixed shortcode resolves to its full region name."""
+        assert "Japan" in handler.parse_tags("Game [Reg-J].rom").regions
+        assert "USA" in handler.parse_tags("Game [Reg-U].rom").regions
+        assert "Netherlands" in handler.parse_tags("Game [Reg-NL].rom").regions
+
+        # A value that is not a shortcode is kept verbatim.
+        assert "PAL" in handler.parse_tags("Game [Reg-PAL].rom").regions
+
     def test_exclude_multi_roms_filters_excluded(self, handler: FSRomsHandler, config):
         """Test exclude_multi_roms filters out excluded multi-file ROMs"""
         roms = ["Game1", "excluded_multi", "Game2", "Game3"]