feat(stable-api): Add RSTRING_END for string end pointer

[ianks]

Summary

Add stable API method for getting a pointer to the end of a Ruby string's contents, equivalent to the C inline function RSTRING_END.

Motivation

RSTRING_END is useful for:

• String iteration from end to beginning
• Bounds checking when manipulating string contents
• Calculating remaining space in string buffers

Currently implemented in Ruby as a static inline function in include/ruby/internal/core/rstring.h.

Ruby Source Reference

// include/ruby/internal/core/rstring.h
static inline char *
RSTRING_END(VALUE str)
{
    char *ptr = RB_FL_TEST_RAW(str, RSTRING_NOEMBED) ?
        RSTRING(str)->as.heap.ptr :
        RSTRING(str)->as.embed.ary;
    long len = RSTRING_LEN(str);
    return &ptr[len];
}

Proposed API

/// Get a pointer to the end of a Ruby string's contents (akin to `RSTRING_END`).
///
/// Returns a pointer to one byte past the last character of the string.
///
/// # Safety
/// This function is unsafe because it dereferences a raw pointer to get
/// access to underlying Ruby data. The caller must ensure that the pointer
/// is valid and points to a T_STRING object.
unsafe fn rstring_end(&self, obj: VALUE) -> *const c_char;

Implementation Notes

• Simple composition: rstring_ptr(obj).add(rstring_len(obj) as usize)
• Same embedded vs heap logic as RSTRING_PTR
• Reference: include/ruby/internal/core/rstring.h:408-422

Checklist

• Add method to StableApiDefinition trait
• Implement for each Ruby version
• Add C fallback in compiled.c
• Add public macro wrapper in macros.rs
• Add tests