Yqr

Added

• Fidelity engines for byte-preserving reads, selectable at runtime with the
  new --engine flag. With --engine noyalib or --engine rust-yaml,
  untouched nodes are emitted as their original source bytes — comments,
  quoting, indentation, and line endings survive, and the identity filter
  reproduces the input byte-for-byte.
• Both fidelity backends now ship in the default build, so --engine noyalib
  and --engine rust-yaml are switchable in one binary without recompiling.
  Build with --no-default-features for a minimal binary that carries neither
  backend (the standard re-serializing pipeline still works; --engine then
  reports the backend as unavailable).
• A backend-agnostic fidelity round-trip harness (tests/fidelity.rs) that
  checks the parse -> emit == input property across backends, one case per
  formatting dimension (comments, blank lines, indent, quote/block/flow style,
  CRLF, BOM, multi-doc, anchors, numbers, key order).
• A shared real-world corpus (tests/corpus/) driving both the validation
  suite and the Criterion benchmarks from one case table (Kubernetes, GitHub
  Actions, Docker Compose, Helm, application config).
• Kubernetes usage guide in the documentation.

Changed

• An unknown --engine value is now diagnosed before any input is read, so a
  typo is reported immediately instead of after consuming stdin or the file.