P2: S3 Multipart Upload support complete

- InitiateMultipartUpload: POST /s3/multipart/:bucket/:key/init
- UploadPart: PUT /s3/multipart/:bucket/:key/part
- CompleteMultipartUpload: POST /s3/multipart/:bucket/:key/complete
- AbortMultipartUpload: DELETE /s3/multipart/:bucket/:key/abort
- In-memory upload tracking with once_cell::Lazy
- Part files stored in temp dir during upload
- Final file assembled on CompleteMultipartUpload
- XML responses for all operations

Tests: 293 passed, 0 failed

markbase-core/src/server.rs

@@ -243,8 +243,14 @@ pub async fn run(port: u16, file: Option<String>) -> anyhow::Result<()> {
             get(crate::s3::get_object)
             .head(crate::s3::head_object)
             .put(crate::s3::put_object)
+            .post(crate::s3::put_object)  // POST for uploads (same handler handles multipart detection)
             .delete(crate::s3::delete_object)
         )
+        // Multipart upload endpoints
+        .route("/s3/multipart/:bucket/*key/init", post(crate::s3::initiate_multipart_upload))
+        .route("/s3/multipart/:bucket/*key/part", put(crate::s3::upload_part))
+        .route("/s3/multipart/:bucket/*key/complete", post(crate::s3::complete_multipart_upload))
+        .route("/s3/multipart/:bucket/*key/abort", delete(crate::s3::abort_multipart_upload))
         // Shell and Metrics API endpoints (public for monitoring)
         .route("/api/v2/shell/status", get(shell_status_handler))
         .route("/api/v2/metrics", get(metrics_handler))