fix: Add per-field serde defaults to Colors for partial config support

[oubiwann]

Problem

When users specify [logging.colors] in their TOML config with only some fields (e.g., just timestamp), serde would deserialize ALL fields of the Colors struct. Unspecified fields would be set to None instead of using the struct's Default implementation values.

This caused twyg to lose colored output for all unspecified color fields.

Example of Broken Behavior

Config file:

[logging.colors]
timestamp = { fg = "HiBlack", bg = "Reset" }

Before this fix:

• timestamp: Some(HiBlack) ✅ (from config)
• level_info: None ❌ (should be HiGreen)
• message: None ❌ (should be Green)
• All other 11 fields: None ❌

Result: Only timestamps were colored, all other log output was plain white text.

Solution

Add #[serde(default = "default_fn")] attribute to each field in the Colors struct, with corresponding default functions. This makes serde apply per-field defaults when fields are missing from the config.

After this fix:

• timestamp: Some(HiBlack) ✅ (from config)
• level_info: Some(HiGreen) ✅ (from default)
• message: Some(Green) ✅ (from default)
• All other fields: Some(...) ✅ (from defaults)

Result: Users can now customize individual colors while keeping defaults for everything else.

Changes

1. Added #[serde(default = "default_*")] to all 13 fields in Colors struct
2. Added 13 default functions (default_timestamp, default_level_info, etc.) matching values from Colors::default()
3. Added test test_colors_partial_deserialization_preserves_defaults to verify the fix

Testing

• ✅ All existing tests pass (147 unit tests + 15 integration tests + 10 doc tests)
• ✅ New test confirms partial config deserialization works correctly
• ✅ Verified with burrow project using twyg (real-world usage)

Example Usage

Users can now do this in their config:

[logging.colors]
# Only customize timestamp - all other colors use twyg defaults
timestamp = { fg = "HiBlack", bg = "Reset" }

Or this:

[logging.colors]
# Customize a few colors
timestamp = { fg = "HiBlack", bg = "Reset" }
level_error = { fg = "HiRed", bg = "Reset" }
arrow = { fg = "HiMagenta", bg = "Reset" }
# All other fields automatically use defaults

Breaking Changes

None. This is a pure bug fix that makes partial configuration work as users would naturally expect.