fix(decoder): validate high bits for first field element in blob decoder

[lavidaloca26]

Summary

Fixes a blob decoding validation gap in BlobDecoder::decode.

Problem

The decoder validated high-order bits for field elements 1..3 (and subsequent rounds), but did not validate field element 0 in the first round (data[0]).

Changes

• Added explicit high-bit validation for the first field element at offset 0.
• Added a regression unit test to ensure malformed blobs with invalid high bits in data[0] return InvalidFieldElement { offset: 0 }.

Impact

Strengthens input validation and prevents malformed blobs from being accepted by the decoder.

[cb-heimdall]

🟡 Heimdall Review Status

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