Formalize overlay semantics in lowering

[riken127]

Title

Formalize and test service overlay semantics

Description

• Service-level fields (env, cmd, entrypoint, etc.) are applied to the last lowered stage.
• This is a critical semantic rule but:
• Not strongly enforced
• Not deeply tested

Acceptance Criteria

• Document overlay behavior clearly
• Add dedicated tests:
• Multiple template uses
• Conflicting overrides
• Ensure deterministic behavior