D065 - cargo doc was failing, and the gate did not build docs

An audit ran cargo doc with -D rustdoc::broken_intra_doc_links and found selfish-pfs would not document at all: a link written as [write] is ambiguous between the module and the write! macro, and rustdoc treats that as an error rather than a warning. Two further links pointed at private items.

Nobody noticed because the gate never built the documentation. In a repository whose durable product is explicitly its prose - a decision log longer than most of its modules, provenance notes on every table, reasoning attached to every constant - the deliverable most worth checking was the one nothing checked.

scripts/gate.sh and CI now build docs with three lints denied. broken_intra_doc_links is the one that matters; the other two catch prose that renders wrong.

And the step was proved to fail before being trusted. A link was deliberately broken, the gate was run and returned 1, the link was restored and it returned 0. That is not ceremony: this script exists because the gate silently swallowed failures twice, and adding an unchecked step to it would have been the same mistake in a new place.