Add square bracket notation for concatenated Morse code transmission

[Copilot]

This PR implements support for square bracket notation [...] to send Morse code characters without inter-character spacing, addressing the need to transmit character combinations like [AR] and [ホレ] as single concatenated patterns.

Problem

In Japanese and European Morse code, certain character combinations are sometimes transmitted as continuous patterns without the standard inter-character spacing. For example:

• AR is normally sent as .- .-. (with space between A and R)
• [AR] should be sent as .-.- (concatenated without space)
• Japanese characters like ホレ can similarly be concatenated as [ホレ]

Solution

The implementation adds bracket parsing to the get_morse_str() function:

1. Bracket Detection: Parses [...] patterns in input strings
2. Pattern Concatenation: Combines individual Morse patterns into single patterns
3. Binary Merging: Efficiently concatenates the binary representations of multiple characters
4. Audio Integration: Updates CW audio generation to handle concatenated patterns correctly

Examples

// Normal transmission with inter-character spacing
get_morse_str("AR".to_string())
// Returns: [(2, 0b01), (0, 0), (3, 0b010)]  // A + space + R

// Concatenated transmission without spacing  
get_morse_str("[AR]".to_string())
// Returns: [(5, 0b01010)]  // Combined A+R pattern

// Works with Japanese characters too
get_morse_str("[ホレ]".to_string())
// Returns: [(6, 0b100111)]  // Combined ホ+レ pattern

Key Features

• Backward Compatibility: All existing functionality remains unchanged
• Multi-language Support: Works with both Latin and Japanese characters
• Audio Generation: CW audio correctly produces shorter transmissions for bracketed text
• Edge Case Handling: Supports empty brackets, unmatched brackets, and mixed content
• Comprehensive Testing: Includes tests for various scenarios and audio verification

The generated audio files confirm the implementation works correctly - bracketed versions produce shorter audio due to eliminated inter-character spacing.

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