Closure migration

Migrate callers before deleting parametric overloads. Prefer value-taking APIs with unified closures. Pair with mojo-syntax (and mojo-gpu-fundamentals for GPU launch code).

Forbidden

Hard ban — do not under any circumstance add @__parameter / @parameter to a nested closure to persist, introduce, or paper over a legacy closure. Not as a migration bridge, not to satisfy a still-capturing API, not to “borrow imm”, not behind a thin *_value wrapper. That is forbidden.