feat(stable-api): Add ENCODING_GET for string/regexp encoding access

[ianks]

Summary

Add stable API methods for accessing Ruby object encodings, equivalent to the C macros ENCODING_GET and ENCODING_GET_INLINED.

Motivation

Encoding handling is critical for text processing in Ruby extensions. Currently, users must call rb_enc_get() which involves additional overhead. Direct access to the encoding index from the object flags would be more efficient.

Proposed API

/// Get the encoding index from a Ruby object (akin to `ENCODING_GET`).
///
/// Works for String, Regexp, and Symbol objects that store encoding
/// information in their flags.
///
/// # 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 an object that has encoding information.
unsafe fn encoding_get(&self, obj: VALUE) -> c_int;

Implementation Notes

• Encoding index is stored in the object's flags (bits 16-23 typically)
• Need to handle ENCODING_INLINE_MAX - larger indices require different handling
• May need to check FL_USHIFT and related flag constants
• Reference: include/ruby/internal/encoding/encoding.h

Checklist

• Add method to StableApiDefinition trait
• Implement for each Ruby version (2.7, 3.0, 3.1, 3.2, 3.3, 3.4, 4.0)
• Add C fallback in compiled.c
• Add public macro wrapper in macros.rs
• Add tests comparing Rust vs C implementation
• Add to show-asm script for performance verification