v1.2.1

No command, flag, exit code, or JSON field changed in this release: nothing
under src/ or packages/ was touched. What changed is how the project
verifies itself. The whole verification gate went from 27 minutes to 8, by
removing work it was doing twice rather than by checking less.

Changed

• The Rust flake checks are crane
  derivations sharing pre-built dependency artifacts, instead of four
  independent buildRustPackage derivations that each recompiled the whole
  dependency graph. Consumers of packages.default or overlays.default get
  the same binary; what changed is that building it no longer runs the test
  suite, which now belongs to checks.<system>.nextest alone. flake.nix
  gains a crane input, and lib.<system>.ciCheckNames is exported so CI can
  derive its job matrix from the checks rather than restating them.
• checks.<system>.msrv stops at cargo check --all-targets instead of
  building and testing the whole workspace under the MSRV toolchain. It
  verifies the same thing — that the declared MSRV still compiles the
  workspace — without a third full run of the test suite.
• The dev profile emits line tables instead of full debug information.
  Backtraces still resolve every frame to file:line; use
  RUSTFLAGS=-Cdebuginfo=2 when step-debugging.
• CI runs one job per flake check instead of one nix flake check for all of
  them, and derives that job list from the flake so a new check cannot silently
  stop being verified.