Set default IncludeDesc fingerprint to zeros

[aqk]

Use an array of zero bytes instead of nil to represent "no fingerprint".
Doing this allows concatenation of an ordered set of signatures to uniquely represent that set.

---

Note

Medium Risk
Changes how include fingerprints are represented and concatenated into the on-disk cache key, which can affect cache hit/miss behavior across builds. Low functional risk beyond caching, but any mismatch in fingerprint population could cause incorrect reuse or unnecessary recompiles.

Overview
Normalizes include fingerprinting for caching. IncludeDesc.fingerprint is changed from a variable-length Vec<u8> to a fixed [u8; 32], with unknown fingerprints represented as 32 zero bytes.

Fingerprint calculation paths now explicitly try_into() the 32-byte tree hash, and the disk cache key builder concatenates fingerprints via extend_from_slice, ensuring deterministic key material even when some includes previously had “no fingerprint”.

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[socket-security]

No dependency changes detected. Learn more about Socket for GitHub.

👍 No dependency changes detected in pull request

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

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[aqk]

@SocketSecurity ignore cargo/zerocopy-derive@0.8.39
@SocketSecurity ignore cargo/cc@1.2.55