Add reproducibility checks for generated XTalk workspaces

[hoebat]

Context

Commit 73a71db9 added approximately 45,000 generated lines across 291 files under packages-gen/js and packages-gen/dart. Generated workspaces are difficult to review manually and can drift from canonical Clojure sources or package-generation rules.

Relevant generator: src-build/xtalk/packages.clj.

Acceptance criteria

• Provide a documented, non-interactive command that regenerates all checked-in XTalk workspaces.
• Ensure generation is deterministic across repeated runs.
• Add CI that regenerates into a clean tree and fails on any diff.
• Validate package manifests, workspace links, and cross-package imports.
• Exclude timestamps, environment-specific paths, and formatting noise from generated output.
• Keep generated files source-derived; fixes must be made in seeds/generators rather than edited directly.