Preserve frontmatter format after leading whitespace

[himanshu748]

Leading whitespace is already stripped by parse(), so JSON and TOML metadata load successfully. However, loads() detects the handler before that normalization and stores None on the post. Dumping the post then silently converts its frontmatter to YAML.

Normalize the input before format detection in loads() so the retained handler agrees with the parser. Regression coverage checks YAML, JSON and TOML with leading blank lines or mixed whitespace through both loads() and text-stream load(), and verifies identical serialization to the unprefixed input.

Validation: the regression failed in all 12 subcases before the fix. Afterward, pytest . --doctest-modules --doctest-glob README.md passes all 68 tests and mypy . passes.

This is separate from #139 (binary stream decoding) and #140 (nested JSON boundaries); neither path is changed here.