fields

Declare `Grouped` as `unsafe trait` to make its soundness invariant
visible.
Make `AnyOutput.type_id` and `member_id` private; add public accessors
and
the `unsafe fn new_bare` constructor.

Make `StructuralData` and `StructuralDataSealed` generic over a program
collector type to bypass the orphan rule for `group_structural!`. This
enables external types to implement `StructuralData<crate::ThisProgram>`
without violating coherence.

Implement `RenderResult::append_other()` to merge buffered content and
immediate output behavior from one result into another. Add the
`r_append!` macro with both explicit buffer (`dst, src`) and implicit
buffer (`src`) usage forms, re-exported from `mingling::prelude`.

Remove `Deref<Target=str>` and `render_text:String` in favor of
`render_buffer:Vec<(String, RenderResultMode)>` and an
`immediate_output`
flag. Add `RenderResultMode` enum, `r_eprint!`/`r_eprintln!` macros, and
new buffer manipulation methods (`append_to_buffer`, `eprint`, etc.).
Update `Display` to trim output and remove trailing newline. Migrate all
callers and tests away from `Deref` usage.

Use Option for load_pathf_map return type and emit a descriptive
compile_error when the type mapping file cannot be loaded, guiding users
to set up build.rs correctly

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>`.

Move routing methods exclusively to the `Routable` trait and update
all macro-generated code to reference `Routable` instead of `Grouped`

Restructure flat module layout into logical directories (attr/, derive/,
func/, systems/, extensions/, utils.rs). Tighten internal function
visibility to pub(crate). All public API signatures remain unchanged.