Cross-cutting: keep-core RNG, Bitcoin/PSBT, and CI hardening

[kwsantiago]

Grab-bag: re-run RNG entropy health-check after fork/snapshot, convert remaining panicking random_bytes() in infallible constructors, right-size MAX_STRUCTURED_PAYLOAD_SIZE, wire the bitcoin-sighash structured payload into PSBT signing, emit PsbtAborted on descriptor_hash mismatch, add EsploraChainView HTTP parse-path + regtest test, and fix the duplicate clippy-allow that breaks CI -D warnings.

This groups several tracked sub-tasks; break out individual PRs as you tackle them.