Fix fixed type slices using same underlying array

[Nepomuk5665]

Summary

Fix a bug where decoded fixed type values share the same underlying array as the input buffer and other decoded values, causing unexpected behavior when appending to these slices.

Problem

This is the same issue as #269 but for the fixed type. PR #270 fixes the bytes type but does not address the fixed type which has the identical problem.

When decoding binary fixed types in makeFixedCodec, the nativeFromBinary function returns buf[:size] directly - a subslice of the input buffer. This means:

1. All decoded fixed values share the same underlying array as the input
2. Appending to a decoded slice can overwrite data in other decoded slices
3. Modifications can corrupt the original input buffer

Solution

Allocate a new slice and copy the data instead of returning a subslice:

result := make([]byte, size)
copy(result, buf[:size])
return result, buf[size:], nil

This ensures each decoded fixed value has its own independent backing array.

Testing

Added TestFixedSlicesUseIndependentBackingArrays which:

1. Decodes a record with two fixed fields from a binary buffer
2. Verifies the initial decoded values are correct
3. Appends to one field and verifies the other field and original buffer are unchanged

All existing tests continue to pass.

Related

• Issue Decoded bytes use the same underlying array #269: Decoded bytes use the same underlying array
• PR Fix byte slices using the same underlying array #270: Fix byte slices using the same underlying array (bytes type only)