Name unsupported constructs in Whale diagnostics #885

Motivation

Executed For, compound-assignment and scalar Match examples currently receive this statement or this expression from whale/src/codegen/mod.rs and whale/src/codegen/walk.rs. The location is present, but the operation is unidentified.

Scope

Replace generic catch-all messages with concise names for recognized unsupported AST variants. Preserve E4001, JSON layout and exact source spans. This task changes diagnostics only, not feature support.

Difficulty: easy.

Completion criteria

• Identify for, match, compound assignment and unsupported expression kinds.
• Assert message, source location and unchanged JSON schema.
• Keep lowering failures from publishing output.