Create Human Evidence Endpoint

Record a reader-selected span as file-captured evidence.

A user action, so it authenticates like the other reader-facing routes rather than with require_kernel_caller — and unlike /api/evidence it cannot create notebook-captured variable evidence, whose value must still come from a kernel read.

The claim's content is the verified quote; a reader's note becomes the title, so an agent reading this back never sees an unverified sentence presented as a quotation from the file.

There is a window between the verifying read and the insert in which the file can change. It is deliberately not locked: the quote is re-verified on every read, so a span invalidated in that window simply shows as diverged the first time the claim is displayed.