Phase 3 of the security-polish series. Brings recovery_qr.rs up to
the documentation density of crypto.rs / imgsecret.rs / backup.rs /
tar_safe.rs. No runtime behaviour change: just module-level //! header
explaining the format + KDF domain separation + parameter-pinning
rationale, an ASCII diagram of the 109-byte payload layout pinned by
a static assertion, doc-comments on the four public items, and named
slice-range constants for the offset arithmetic.
production_params() is replaced with a top-level const so the "pinned,
do not change once shipped" property is visible at every use site.
Refs: docs/superpowers/specs/2026-05-04-security-polish-design.md (Phase 3)
Refs: docs/superpowers/reviews/2026-05-04-architecture-review.md (P1.7)
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
