Fix: Only create mismatch directory when files are actually written

[Copilot]

The datarecorder was creating mismatch directories even when no mismatch files needed to be written, resulting in empty folders accumulating on CI systems.

Problem

The determine_mismatch_dir() function immediately created a directory in /tmp/cppmismatch-N/ whenever a mismatch was detected, regardless of whether any files would actually be written to it. This meant that:

• The default mismatch handler (which doesn't write files) still created empty directories
• Any interruption during mismatch handling could leave empty directories behind
• CI systems accumulated unnecessary empty folders over time

Solution

Split the directory path determination from directory creation:

1. Renamed determine_mismatch_dir() to determine_mismatch_path() - now only returns the path without creating directories
2. Enhanced write_data() with lazy directory creation - automatically creates parent directories when files are actually written using std::filesystem::create_directories()

Example

Before this fix:

// Always creates /tmp/cppmismatch-0/ even if no files are written
std::filesystem::path mismatch_dir = determine_mismatch_dir();
// Directory exists but may remain empty

After this fix:

// Only determines the path, no directory created yet
std::filesystem::path mismatch_dir = determine_mismatch_path();
// Directory is created only when write_data() is called
write_data(mismatch_dir / "diff.html", html_content);

This ensures mismatch directories are created only when they actually contain files, eliminating the accumulation of empty directories on CI systems.

Fixes #4.

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