Append a ledger record

Requires a workspace-bound API key with ledger:write and an owner, admin, or member role. Appends one designator, observation, or claim. A designator with the same kind, namespace, and normalized value is reused (200, created false). Every claim cites at least one observation. The first line of a claim key supersedes nothing; every later line, including a retraction, must supersede the current head, or the request fails with 409. Verification must be one the cited evidence supports: cross-linked and corroborated need retrievals of two different pages; analytic means the only support is an analyst note. Other rule failures return 400 with the rule codes in detail. Send an Idempotency-Key header to make retries safe.