Please address the comments from this code review:

## Overall Comments
- The new download_md5 logic drops the previous region‐based MD5 URL derivation—please verify that custom or region-specific tar.bz2 URLs still resolve their checksums correctly.
- Using `endswith` on the download URL to detect file types can be brittle (e.g., uppercase extensions or query params); consider parsing the path or MIME type for more robust format detection.
- You might extract the shell‐command construction for different formats into a small helper or strategy class to simplify adding future formats and make the code more maintainable.

## Individual Comments

### Comment 1
<location> `src/downloader.py:284-291` </location>
<code_context>

 def download_index() -> str:
-    output_file = "photon-db-latest.tar.bz2"
     download_url = get_download_url()
+    if download_url.endswith(".jsonl.zst"):
+        output_file = "photon-data-dump.jsonl.zst"
</code_context>

<issue_to_address>
**suggestion:** The new download_md5 logic assumes all download URLs can be suffixed with '.md5'.

If the server does not support '.md5' suffixes for all URLs, this logic may fail. Consider adding error handling or validating the URL format before appending the suffix.

Suggested implementation:

```python
def download_md5(download_url: str) -> str:
    """
    Attempts to download the MD5 file for the given download_url.
    Handles cases where the .md5 file is not available.
    Returns the path to the downloaded MD5 file, or raises an exception if not found.
    """
    import requests

    md5_url = download_url + ".md5"
    try:
        response = requests.get(md5_url, timeout=10)
        response.raise_for_status()
        md5_file = os.path.join(config.TEMP_DIR, os.path.basename(md5_url))
        with open(md5_file, "wb") as f:
            f.write(response.content)
        return md5_file
    except requests.exceptions.HTTPError as e:
        if response.status_code == 404:
            raise FileNotFoundError(f"MD5 file not found for URL: {md5_url}")
        else:
            raise
    except Exception as e:
        raise RuntimeError(f"Failed to download MD5 file: {e}")

```

You will need to update any calls to `download_md5()` to pass the `download_url` as an argument.  
If you want to validate the URL format before appending `.md5`, you can add a check to ensure the URL is a valid file download URL (e.g., ends with a known extension).
</issue_to_address>

### Comment 2
<location> `src/filesystem.py:24-27` </location>
<code_context>
     if not os.path.exists(config.TEMP_DIR):
         logging.debug(f"Creating temp directory: {config.TEMP_DIR}")
         os.makedirs(config.TEMP_DIR, exist_ok=True)
+    if index_file.endswith(".jsonl.zst"):
+        install_command = f"zstd --stdout -d {index_file} | java -jar /photon/photon.jar -nominatim-import -import-file - -data-dir {config.TEMP_DIR} {config.BUILD_PHOTON_PARAMS}"
+    else:
+        install_command = f"lbzip2 -d -c {index_file} | tar x -C {config.TEMP_DIR}"

</code_context>

<issue_to_address>
**suggestion:** Extraction logic for '.jsonl.zst' files assumes zstd and Java are available and compatible.

Consider checking for zstd and Java availability at startup, and enhance error messages to guide users on resolving missing or incompatible dependencies.

```suggestion
    def check_executable_available(executable_name):
        from shutil import which
        if which(executable_name) is None:
            raise RuntimeError(
                f"Required executable '{executable_name}' not found in PATH. "
                f"Please install '{executable_name}' and ensure it is available in your system PATH."
            )

    if index_file.endswith(".jsonl.zst"):
        check_executable_available("zstd")
        check_executable_available("java")
        install_command = f"zstd --stdout -d {index_file} | java -jar /photon/photon.jar -nominatim-import -import-file - -data-dir {config.TEMP_DIR} {config.BUILD_PHOTON_PARAMS}"
    else:
        install_command = f"lbzip2 -d -c {index_file} | tar x -C {config.TEMP_DIR}"
```
</issue_to_address>