fix(if-else): stop legacy fallback on case miss

[WH-2099]

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Related Issue

Closes #43

Summary

• stop falling back to deprecated top-level conditions when cases are present but none match
• treat cases=[] as an explicit new-shape configuration instead of a legacy signal
• restructure the if_else tests around runtime behavior, shape priority, short-circuiting, warnings, and variable mapping

Checklist

• This pull request links the issue it resolves or advances
• This pull request title follows Conventional Commits, and any breaking change is marked with !
• If CLA Assistant prompted me, I signed CLA.md in the pull request conversation