fix: name the checksum algorithm in PutObject/UploadPart mismatch errors

hash.ChecksumMismatch now carries the requested algorithm (CRC32, CRC32C,
SHA1, SHA256, CRC64NVME), and toAPIError renders it S3-style:

  The CRC32 you specified did not match the calculated checksum.

instead of the generic "The provided 'x-amz-checksum' header does not match
what was computed.", for all four mismatch construction sites (streaming
reader, trailing-checksum decode, and both Checksum.Matches paths). Mirrors
the wording already shipped for CompleteMultipartUpload in #225.

Fixes #226

internal/hash/checksum.go

@@ -584,15 +584,17 @@ func (c Checksum) Matches(content []byte, parts int) error {
 	sum := hasher.Sum(nil)
 	if c.WantParts > 0 && c.WantParts != parts {
 		return ChecksumMismatch{
-			Want: fmt.Sprintf("%s-%d", c.Encoded, c.WantParts),
-			Got:  fmt.Sprintf("%s-%d", base64.StdEncoding.EncodeToString(sum), parts),
+			Algorithm: c.Type.String(),
+			Want:      fmt.Sprintf("%s-%d", c.Encoded, c.WantParts),
+			Got:       fmt.Sprintf("%s-%d", base64.StdEncoding.EncodeToString(sum), parts),
 		}
 	}
 
 	if !bytes.Equal(sum, c.Raw) {
 		return ChecksumMismatch{
-			Want: c.Encoded,
-			Got:  base64.StdEncoding.EncodeToString(sum),
+			Algorithm: c.Type.String(),
+			Want:      c.Encoded,
+			Got:       base64.StdEncoding.EncodeToString(sum),
 		}
 	}
 	return nil

internal/hash/errors.go

@@ -74,8 +74,11 @@ func (e SizeMismatch) Error() string {
 
 // ChecksumMismatch - when content checksum does not match with what was sent from client.
 type ChecksumMismatch struct {
-	Want string
-	Got  string
+	// Algorithm carries the requested checksum type (e.g. "CRC32") so API
+	// error rendering can name it, matching S3's wording.
+	Algorithm string
+	Want      string
+	Got       string
 }
 
 func (e ChecksumMismatch) Error() string {

internal/hash/reader.go

@@ -302,13 +302,14 @@ func (r *Reader) Read(p []byte) (int, error) {
 				r.contentHash.Encoded = r.trailer.Get(r.contentHash.Type.Key())
 				r.contentHash.Raw, err = base64.StdEncoding.DecodeString(r.contentHash.Encoded)
 				if err != nil || len(r.contentHash.Raw) == 0 {
-					return 0, ChecksumMismatch{Got: r.contentHash.Encoded}
+					return 0, ChecksumMismatch{Algorithm: r.contentHash.Type.String(), Got: r.contentHash.Encoded}
 				}
 			}
 			if sum := r.contentHasher.Sum(nil); !bytes.Equal(r.contentHash.Raw, sum) {
 				err := ChecksumMismatch{
-					Want: r.contentHash.Encoded,
-					Got:  base64.StdEncoding.EncodeToString(sum),
+					Algorithm: r.contentHash.Type.String(),
+					Want:      r.contentHash.Encoded,
+					Got:       base64.StdEncoding.EncodeToString(sum),
 				}
 				return n, err
 			}