feat(core)!: replace RenderResult with buffered output model

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.

2 files changed, 74 insertions, 0 deletions

diff --git a/mingling_macros/src/func/r_print.rs b/mingling_macros/src/func/r_print.rs
index e81b544..0f77b2b 100644
--- a/mingling_macros/src/func/r_print.rs
+++ b/mingling_macros/src/func/r_print.rs
@@ -60,3 +60,11 @@ pub(crate) fn r_println(input: TokenStream) -> TokenStream {
 pub(crate) fn r_print(input: TokenStream) -> TokenStream {
     expand_print(input, "print")
 }
+
+pub(crate) fn r_eprintln(input: TokenStream) -> TokenStream {
+    expand_print(input, "eprintln")
+}
+
+pub(crate) fn r_eprint(input: TokenStream) -> TokenStream {
+    expand_print(input, "eprint")
+}
diff --git a/mingling_macros/src/lib.rs b/mingling_macros/src/lib.rs
index 56ed999..5e7a40f 100644
--- a/mingling_macros/src/lib.rs
+++ b/mingling_macros/src/lib.rs
@@ -1412,6 +1412,72 @@ pub fn r_print(input: TokenStream) -> TokenStream {
     func::r_print::r_print(input)
 }
 
+/// Prints text to a `RenderResult` buffer (standard error style), with a trailing newline.
+///
+/// This macro works identically to `r_println!` but conceptually targets
+/// "error output" — it writes into a `RenderResult` buffer with a trailing newline.
+///
+/// # Implicit buffer (inside `#[buffer]` functions)
+///
+/// ```rust,ignore
+/// use mingling::macros::{buffer, r_eprintln};
+///
+/// #[buffer]
+/// fn render() {
+///     r_eprintln!("Error: {}", err_msg);
+/// }
+/// ```
+///
+/// # Explicit buffer
+///
+/// Pass a `RenderResult` variable as the first argument:
+///
+/// ```rust,ignore
+/// use mingling::macros::r_eprintln;
+/// use mingling::RenderResult;
+///
+/// let mut r = RenderResult::new();
+/// r_eprintln!(r, "error: {}", 42);
+/// assert_eq!(&*r, "error: 42\n");
+/// ```
+#[proc_macro]
+pub fn r_eprintln(input: TokenStream) -> TokenStream {
+    func::r_print::r_eprintln(input)
+}
+
+/// Prints text to a `RenderResult` buffer (standard error style), without a trailing newline.
+///
+/// This macro works identically to `r_print!` but conceptually targets
+/// "error output" — it writes into a `RenderResult` buffer without a trailing newline.
+///
+/// # Implicit buffer (inside `#[buffer]` functions)
+///
+/// ```rust,ignore
+/// use mingling::macros::{buffer, r_eprint};
+///
+/// #[buffer]
+/// fn render() {
+///     r_eprint!("Error: ");
+///     r_eprintln!("something went wrong");
+/// }
+/// ```
+///
+/// # Explicit buffer
+///
+/// ```rust,ignore
+/// use mingling::macros::r_eprint;
+/// use mingling::RenderResult;
+///
+/// let mut r = RenderResult::new();
+/// r_eprint!(r, "error: ");
+/// r_eprintln!(r, "42");
+/// assert_eq!(&*r, "error: 42\n");
+/// ```
+#[proc_macro]
+pub fn r_eprint(input: TokenStream) -> TokenStream {
+    func::r_print::r_eprint(input)
+}
+
 /// Derive macro for automatically implementing the `Grouped` trait on a struct.
 ///
 /// The `#[derive(Grouped)]` macro: