refactor: remove unnecessary path computation from export_to_gpx early return

IMPROVES SEPARATION OF CONCERNS
- export_to_gpx() should only export data, not compute paths for messaging
- Removed unnecessary path computation when gps_coordinates is empty
- CLI already uses compute_export_paths() for message generation
- Export functions stay focused on their core responsibility

BENEFITS
- Cleaner library API: export functions don't couple to CLI messaging needs
- Better separation of concerns: path computation is CLI responsibility
- More reusable: export functions can be used by other tools without side effects
- Supports goal: Moving to shared libraries for both CLI and crate usage

IMPLEMENTATION
- Removed ~20 lines of path computation code from export_to_gpx early return
- CLI message generation unchanged (already uses compute_export_paths)
- export_to_event was already correct (had early return without path computation)

VERIFICATION
✅ cargo fmt --all -- --check    : PASS
✅ cargo clippy --all-targets    : PASS
✅ cargo test --verbose          : PASS (37 tests)
✅ cargo build --release         : PASS
✅ CSV output vs master          : PASS (87/87 files identical)
✅ GPX/event messaging            : PASS (messages still displayed correctly)

ALIGNMENT WITH GOALS
✅ De-duplication: Further reduced code duplication
✅ Shared libraries: Export functions now cleaner library APIs
✅ Better separation: Export functions don't depend on CLI messaging

Author: nerdCopter

src/export.rs

@@ -359,25 +359,6 @@ pub fn export_to_gpx(
     log_start_datetime: Option<&str>,
 ) -> Result<()> {
     if gps_coordinates.is_empty() {
-        // Compute path for consistency even though we're not writing anything
-        // (allows CLI to log a consistent message)
-        let base_name = input_path
-            .file_stem()
-            .and_then(|n| n.to_str())
-            .unwrap_or("blackbox");
-
-        let output_dir = export_options
-            .output_dir
-            .as_deref()
-            .map(Path::new)
-            .unwrap_or_else(|| input_path.parent().unwrap_or(Path::new(".")));
-
-        let log_suffix = if total_logs > 1 {
-            format!(".{:02}", log_index + 1)
-        } else {
-            String::new()
-        };
-        let _gpx_path = output_dir.join(format!("{}{}.gps.gpx", base_name, log_suffix));
         return Ok(());
     }