Add comprehensive test coverage for stty:

Unit tests (src/uu/stty/src/stty.rs):
- Flag struct methods and builder pattern (7 tests)
- Control character parsing and formatting (19 tests)
- All combination settings expansion (26 tests)
- Termios modification functions (13 tests)
- String-to-flag/combo/baud parsing (19 tests)
- TermiosFlag trait implementations (5 tests)
- Helper and utility functions (10 tests)
- Trait implementations (2 tests)

Integration tests (tests/by-util/test_stty.rs):
- Help and version output validation (2 tests)
- Invalid argument handling (3 tests)
- Control character overflow validation (2 tests)
- Grouped flag removal validation (1 test)
- File argument error handling (1 test)
- Conflicting print modes (1 test)
- Additional TTY-dependent tests (6 tests, ignored in CI)

Unit test coverage improved from 0% to 43.76% (207/473 lines).
Integration tests validate argument parsing and error handling.

Addresses uutils#9061

- Added 11 essential unit tests for complex internal functions:
  * Control character parsing (string_to_control_char)
  * Control character formatting (control_char_to_string)
  * Combination settings expansion (combo_to_flags)
  * Terminal size parsing with overflow handling (parse_rows_cols)
  * Sane control character defaults (get_sane_control_char)

- Added 16 integration tests for command behavior:
  * Help/version output validation
  * Invalid argument handling
  * Control character overflow validation
  * Grouped flag removal validation
  * File argument error handling
  * Conflicting print modes
  * TTY-dependent tests (marked as ignored for CI)

Unit tests focus on complex parsing logic that's difficult to test via
integration tests. Integration tests validate actual command behavior.

Coverage improved from 0% to 43.76% (207/473 lines).

Fixes uutils#9061

- Add unit tests for parse_rows_cols() with edge cases and wraparound
- Add unit tests for string_to_baud() with platform-specific handling
- Add unit tests for string_to_combo() with all combo modes
- Add 17 integration tests for missing arguments and invalid inputs
- Enhance test_invalid_arg() with better error message assertions
- Update coverage script for improved reporting

Coverage improved from 22.26% to 23.14% regions.

- Add #[derive(Debug, PartialEq)] to AllFlags enum
- Add PartialEq to Flag struct derives
- Enables assert_eq! macro usage in unit tests

- Add spell-checker:ignore comments for test data (notachar, notabaud, susp)
- Add spell-checker:ignore comments for French error strings (Valeur, entier, invalide)
- Fixes cspell validation without modifying global config

The PartialEq derive was being compiled into release builds even though
it's only used in test code. This caused a 3.33% performance regression
in the du_human_balanced_tree benchmark due to increased binary size
affecting CPU cache efficiency.

Changes:
- stty.rs: Gate PartialEq derive on Flag<T> with #[cfg_attr(test, derive(PartialEq))]
- flags.rs: Gate PartialEq derive on AllFlags enum with #[cfg_attr(test, derive(PartialEq))]

This eliminates the performance regression while keeping all test code
functional and unchanged.