V-L2-F2: implement the JSON sidecar storage backend (split from #45)

[hyperpolymath]

Split out of #45 per its acceptance note ("json sidecar storage is either implemented or removed from manifest (separate issue)").

[sidecar].storage advertised sqlite | json but only sqlite has code; #45 stops silently emitting SQLite DDL for a json store and rejects storage="json" with a pointer here. This issue tracks actually implementing a JSON sidecar (append-only JSON document store mirroring the verisimdb_* tables) OR formally dropping json from the manifest schema + validation if a JSON store is not wanted.

Acceptance:

• Decision: implement JSON store vs drop the option from the manifest enum + docs + validate
• If implement: a JSON storage module with the same octad-dimension coverage as the sqlite path; generate/drift/gc honour it
• If drop: storage enum, SidecarConfig docs, manifest template, and validate/doctor messaging no longer mention json