Publish Index Head

Publishes one SPEC.md §HEAD-1 head-commit publication event for an index or benchmark's per-stream publication history, so an administrator's published history (art-646-compile-rebalance-evidence-pack results, or art-647-record-index-correction events) becomes a sequence-numbered, signer-continuous chain instead of a series of unlinked artifacts a reviewer must independently discover and order, mirroring NAV-LINEAGE-BUILD-SPEC.md §3 and art-649-publish-model-risk-head, applied to an index/benchmark administrator's stream (INDEX-LINEAGE-BUILD-SPEC.md §5). HARD FENCE: this node never accepts or handles private key material, the caller signs the head-commit off-node via chaingraph/kernels/_head.mjs's own buildHead/signHead and separately runs its own Ed25519 verification (again via _head.mjs's verifyHeadProof/verifyChain) before calling this node. signature_valid and chain_valid are the caller's own verification claim, asserted and digested into this receipt, never independently re-derived by this node (the real zkVM guest has no WebCrypto at all, so an in-kernel Ed25519 verify result would not be reproducible across this repo's required execution environments). The one field this node DOES independently recompute is head_hash (pure SHA-256/JCS over the caller-supplied head, never trusted as a caller-asserted value). Written for administrators in the post-Regulation-(EU)-2025/914 BMR scope (critical/significant benchmarks) and for SEBI-regulated benchmark administrators; this node makes no BMR/SEBI in-scope determination of its own and makes no claim of BMR or SEBI compliance itself.