Add comprehensive unit tests for core modules

[Copilot]

The codebase lacked unit tests for critical functions in configuration loading, state management, and utility functions. Test coverage increased from 20 to 72 tests.

Changes

Created missing build infrastructure:

• Added Cargo.toml with dependencies and test framework
• Updated .gitignore to allow Cargo.toml while excluding other TOML files

config.rs (24 new tests):

• TOML parsing with valid/invalid/missing files
• Environment variable overrides for all 15+ configuration options
• Default value initialization for all structs
• Edge cases (zero values, empty strings)

main.rs (11 new tests):

• Status file parsing with malformed/partial/missing data
• Path expansion (~, absolute, relative paths)
• Control file name generation
• Temperature optimization logic (first-time vs. cached values)

state.rs (14 new tests):

• State persistence round-trip (save/load)
• Temperature interpolation calculations
• All easing functions (linear, ease_in, ease_out, ease_in_out) with edge values
• State age calculations
• Path expansion and directory creation

transition.rs (7 new tests):

• Zero-duration transition handling
• Transition restart on target change
• Progress calculation edge cases
• Initialization state verification

All tests pass. CodeQL scan shows 0 vulnerabilities.

Original prompt

Analyze the code and create unit tests where approrpiate.

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