nx_install_hash.nx

nx_install_hash.nx -- compute install_hash from NxInstallPlan via canonical serialization + SHA-256. SA-7 partial milestone of NISHI_SELF_ASSEMBLY_ROADMAP.md. The install_hash is the racing-line "ProvenanceLink" per [[feedback-end-to-end-bit-traceability-architecture]]: every install attestation carries a hash that future audits can reproduce by replaying the selector against the recorded inputs. install_hash = SHA-256(canonical_serialize(plan)) Canonical serialization rules: 1. Magic tag "NXIPLN01" (8 bytes) so plans of different schema versions cannot collide 2. Every fixed i64 field written little-endian, in declared struct order 3. EXCLUDED: canary_pre + canary_post (tamper-detect, not content); ts_us (timestamp varies across runs but inputs same) 4. selected_variant_ids array written after n_selected (which acts as the length prefix), each i64 little-endian Reproducibility gate (the SA-7 verification): same inputs (probe + calibration + policy + selection) produce bit-equal install_hash. Differences in ts_us or canaries do NOT affect the hash (excluded by design). Sensitivity gate: any change to a content field (probe, calibration, policy, selection) produces a different install_hash. This makes the install_hash a sensitive integrity check, not just a deterministic value. Refuses to hash a tampered plan: nx_install_plan_is_valid must return 1 before serialization; otherwise returns NX_HASH_BAD_PLAN. Canary tamper is a substrate-honesty signal, not something to silently hash over. Deferred per roadmap: - in-toto link metadata serialization with the hash chained to source.nx file hashes (SA-7 remainder) - Rekor-class append-only audit log local-by-default (SA-7