Add expression macro for ad-hoc error construction (anyhow! parity)

[RAprogramm]

anyhow 1.x provides the anyhow! macro that builds an error value as an expression from a format string, an existing error, or both. masterror only exposes statement-form macros (ensure!, fail!).

Add an expression macro (e.g. error!(AppErrorKind::Validation, "bad input: {value}")) that constructs an AppError inline, supporting:

• format-string interpolation (implicit captures like format!)
• wrapping an existing impl Error + Send + Sync value
• optional AppCode/kind selection with a sensible default

This closes the last ergonomic gap with anyhow for application code.