Fix mixed value type decoding in sds-convert

[ricardoasouz]

Fixes #271

The original IndexError reported in #271 is no longer reproducible on the current main branch, but the remaining mixed-type conversion issue persists.

When a record contains values with different sizes, such as:

• double
• uint32_t
• uint32_t
• uint32_t
• uint32_t

The current decoder processes each channel independently, assuming a uniform stride. This causes the byte stream to become misaligned across channels.

For example, the SDS data referenced in #271 decodes correctly as:

For example, the SDS data referenced in #271 decodes correctly as:

confidence=0.7162540555000305
x=143
y=9
w=40
h=55

while the current converter produces:

confidence=0.7162540555000305
x=1072098189
y=143
w=9
h=40

This change decodes each complete sample using the combined metadata format, then distributes the decoded values to the corresponding channels. Standard-size packing is used without native alignment padding.

Validation:

• Reproduced the incorrect conversion using the SDS-Examples data referenced in [Bug]: sds-convert does not properly handle different value types #271.
• Verified the corrected output against an independent struct-based decode.
• Added regression coverage for mixed-size value types.
• Added compatibility coverage for equal-size value types.
• Added coverage for per-channel scale and offset handling.
• All 3 unit tests pass with unittest discovery.