BReg docs: the apply-time Evidence and target precondition guard has no configure coverage

Start with docs/site/src/content/docs/configure/breg-change-control.mdx and compare its existing precondition coverage with the runtime references in crates/registry-breg/src/request_prepare.rs, change_request.rs, contract.rs, and action_evidence_contracts.rs. Use the issue's rg searches to verify the current gaps; done means the page documents application preconditions, manual-only behavior, ceilings, atLeast/atMost bounds, and the distinct failure contracts.

What

A change request can declare apply-time preconditions: changeRequest.application.preconditions with request[] predicates, targets[] guards and evidence[] requirements, evaluated at manual application rather than at submit or approval. None of it has a page on the documentation site. The configure/breg-change-control.mdx page documents action preconditions (target ETags on an invoke) and says nothing about the application block, so a reader who searches for "preconditions" finds the wrong feature and concludes the apply-time guard does not exist.

The atMost bound on an Evidence or predicate requirement is undocumented anywhere on the site, in either the change-request or the action-evidence form.

Evidence

• Runtime surface: crates/registry-breg/src/request_prepare.rs lines 320 to 442 walks plan.application.preconditions.request, .targets and .evidence and freezes them as FrozenApplicationPreconditions. crates/registry-breg/src/change_request.rs line 747 validates the block, line 96 emits change_request.application.preconditions_manual_only, and line 773 emits the ceiling refusal "application preconditions exceed the finite target, Evidence, predicate, or byte ceiling".
• at_most (serialized atMost) is declared twice in crates/registry-breg/src/contract.rs, lines 599 and 659, and consumed in crates/registry-breg/src/action_evidence_contracts.rs lines 150 to 166 and crates/registry-breg/src/change_request.rs lines 986, 1106 and 1159.
• Docs coverage: rg -i precondition docs/site/src/content/docs/configure/breg-change-control.mdx matches only lines 281, 515 and 534, all about target ETags on actions and on request target admission. rg -i atmost docs/site/src/content returns nothing.

Proposed fix

Add a section to the change-control configure page for the application precondition block: the three kinds, the manual-application-only rule and its diagnostic, the finite ceilings, and the atLeast and atMost bounds on a requirement. Include the failure contract in the same place, distinguishing 412 precondition.failed (a guard no longer holds) from 503 action.evidence_failed (the Evidence check could not be completed), since the caller's recovery differs.

Found while

Extracting reusable App Kit skills against Registry Stack v0.32.0.