Make fuzz targets deterministic

Disable the std feature for the lightning dependency in fuzz builds.
The no-std code paths already use highest_seen_timestamp instead of
SystemTime::now() for time, and use zeroed SipHash keys for
deterministic hash table ordering. This makes fuzz test cases
reproduce consistently without any cfg(fuzzing) changes to the
lightning crate.

AI tools were used in preparing this commit.

Author: joostjager

fuzz/Cargo.toml

@@ -18,7 +18,7 @@ libfuzzer_fuzz = ["libfuzzer-sys"]
 stdin_fuzz = []
 
 [dependencies]
-lightning = { path = "../lightning", features = ["regex", "_test_utils"] }
+lightning = { path = "../lightning", default-features = false, features = ["regex", "_test_utils"] }
 lightning-invoice = { path = "../lightning-invoice" }
 lightning-liquidity = { path = "../lightning-liquidity" }
 lightning-rapid-gossip-sync = { path = "../lightning-rapid-gossip-sync" }