spb/metrika Public

- fix: script-command verbs were routed AFTER parsing, which rejects
  unknown verbs — .zyq commands could never run; they now route before
  the parser with raw macro arguments
- .zyq scripts: ado-style 'args name…' declaration with backtick macro
  substitution (`0', `k', `name'); scratch-variable scripts verified
  end-to-end against the built-in path
- ZQPluginRegistry: verb-keyed native plugin registry; verbs shadowing
  built-ins rejected at session start; parsed commands validated against
  the declared ZQSyntaxSpec (varlist/if/in/weights/options)
- mutation gating: plugins return a replacement dataset via
  ZQResult.replacementFrame, installed only under 'mutates: true' — a
  non-mutating plugin returning one is an error and the dataset is
  untouched (ZQContext is now a plain value, no writable back-channel)
- ZScorePlugin ships as the reference native plugin, registered by the
  app ('zscore varname' -> standardized z_varname)
- CLAUDE.md: document the mlx-swift build quirks (xcodebuild for GPU
  tests, -skipPackagePluginValidation everywhere)
- 87 tests green (swift test and xcodebuild with GPU suites); app builds

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>