ContractBackend trait — proof-lane rendering abstraction.
Renders parsed [xpile_contracts::Contract] into LaTeX (math +
theorem environments), Lean 4 theorem text (with @[xpile_contract]
attribute carrying the verbatim contract ID), or mdBook.
Architectural invariants codified in
contracts/xpile-contract-backend-trait-v1.yaml. The citation bridge
uses format-native structured constructs parseable by the host
elaborator/parser, NOT regex over body text. See
docs/specifications/sub/contract-backend-trait.md §"Citation bridge".