Plan: Slice 5 Platonik session adapter - #30

Summary

Stacked on #24. Adds kb/plans/valhalla-platonik-session-adapter.md, the plan the witness platform plan deferred for promotion-gates slice 5: "Platonik runs through an optional Valhalla session adapter and a receiver independently verifies the result."

• An optional std crate crates/vhalla-game-platonik (never a default platform dependency) that maps the games plan's GameManifest, SessionOpen, GameEvent, Checkpoint, and Settlement onto vhalla-witness task manifests, assignments, case events, state hashes, and receipts, with Platonik experiment_hash and result_hash carried as opaque inner artifact ids.
• Host-ordered sessions over vhalla-ledger with explicit, non-permissionless authority: the host orders and includes, every receiver re-derives every root, equivocation and omission are retained evidence, and settlement is a signed result or an explicit unresolved fork.
• A two-phase seed bind (commit, host-signed close, reveal) so no party, including the host, can choose seeds; pause, replace, and resume as replay-from-genesis with a chained trace head; bounded artifact requests for large traces; verification budgets per session and per event.
• Discovery spikes with pass and fail criteria, staged delivery naming the first PR's scope, a threat model, and a decision record.

Produced by a judge panel over three angled architectures and revised through three adversarial review rounds (six lenses, two skeptics per finding); the applied findings are listed in the plan's review section.

Gates

Plan-only change. kb check --root kb passes with the pre-existing advisory orphan.

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