All notable changes to this project will be documented in this file.

• previous_char_boundary now clamps out-of-bounds indices to self.len(), matching next_char_boundary.
• Split the internal sealing traits by purpose and replaced the Deref-based StrExt implementation with explicit impls for str and String.
• Removed the set convenience method from StringExt.
• Moved the test suite out of src/lib.rs into tests/lib.rs.
• Standardized and expanded the crate, API, and README documentation.

• longest_common_substring now compares substrings by char length instead of UTF-8 byte length.
• Corrected the documented space complexity of longest_common_substring to match the implementation.

• Added byte_frequencies for counting byte occurrences in a string's UTF-8 representation.
• Added truncate_to_chars and truncate_to_chars_in_place for char-count-based truncation.
• Added pad_start_to, pad_end_to, center_to and their in-place variants for exact char-width padding with a single fill char.
• Added strip_prefix_in_place and strip_suffix_in_place as mutable counterparts to the standard prefix/suffix stripping methods.
• Added common_prefix and common_suffix for borrowed shared prefix/suffix queries.
• Added regression coverage for Unicode behavior in longest_common_substring.
• Added edge-case coverage for no-op behavior in shift, shift_in_place, expand_tabs, expand_tabs_in_place, and replace_in_place.
• Added panic coverage for invalid indices in shift and shift_in_place.
• Added method-level doctest examples and extra API behavior notes.