Use an array of zero bytes instead of nil to represent "no fingerprint"

[aqk]

Doing this allows concatenation of an ordered set of signatures to uniquely represent that set.

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Note

Medium Risk
Touches dependency fingerprinting and cache-key generation, so mistakes could cause incorrect cache hits/misses or reduced cache effectiveness despite the change being mechanically straightforward.

Overview
Standardizes include fingerprints to a fixed 32-byte SHA-256 digest. IncludeDesc.fingerprint changes from Vec<u8> (with “no fingerprint” as empty) to [u8; 32] (with “no fingerprint” as all-zero bytes), and all include/namespace/embed paths now populate this field accordingly.

This updates cache key construction in diskcache to concatenate fixed-size fingerprints deterministically, and adds explicit try_into() conversions where SHA-256 results are produced (e.g., add_main_fingerprint, module/import hashing).

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