pipeline error routing

macros

Reintroduce `r_print!` and `r_println!` macros as public exports, now
supporting both explicit buffer argument and implicit `#[buffer]` attr.
Add `#[buffer]` attribute macro that wraps unit-returning functions to
produce a `RenderResult` with an automatically injected buffer variable.
Relax `RenderResult::print()` and `println()` to accept `impl
AsRef<str>`.

Implement an `extensions` module in `mingling_macros` that allows
attribute macros to accept extension identifiers (e.g., `routeify`)
which are re-dispatched as outer attributes before the core macro
logic runs, enabling extensibility without modifying existing macros