Validate conditional branch structure when requested

[fzdwx]

When a model puts $then and $else inside $cond, structural validation currently accepts the spec even though the conditional resolver expects sibling branches. Hosts can then render an object as a label or write an unintended value instead of requesting a repair.

Add the opt-in checkConditionalBranches option to validateSpec. It walks props, event actions and watchers, including nested values, and reports invalid_conditional_branches with the expression’s JSON Pointer in both issue.path and the formatted message. The default remains off, initial state is excluded, and no expression is evaluated or rewritten. README, API documentation and the core skill describe the option.

Validation: all 40 spec-validator tests pass, the existing core tsconfig.check.json check passes, and ESM/CJS/declaration builds pass. The whole-workspace type-check could not run in this isolated checkout because its workspace dependencies are not installed; the package-manager attempt triggered an automatic install and was stopped.

[vercel]

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