feat: merge-train/spartan-v5 (#24596) [clean pick -m 1]

Clean forward-port of #24596 (merge-train
spartan-v5, merge d2f4c4d, parents 42eea48..1737dfb) onto
cb/private-port-next-7. Source base 42eea48 is #24576's merge (block 4),
so it stacks directly on block 4.

Zero conflicts; tree touches exactly the 5 source-diff files: one code change
(prover-client checkpoint-sub-tree-orchestrator empty-addTxs log demoted
warn -> verbose, #24593) + 4 docs (new yarn-project/THREAT_MODEL.md and
p2p/reqresp/slasher READMEs, #24465). No noir/circuit/pinned surfaces.

Bundles sub-PRs #24593 #24465.

Tracker: AztecProtocol/aztec-packages-private#588

Author: AztecBot