Fix help padding and preserve caller stream formatting

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Printing help to a stream previously configured with std::setfill('0') pads argument names with zeros (for example, mode000000000). The help formatter also leaves std::left set on the caller's stream.

Use space fill and left alignment while rendering help, then restore the original formatting flags and fill character with a local RAII guard, including when writing throws. This leaves existing width behavior unchanged. The regression tests cover positional/optional arguments, multiline help, subcommands, subsequent formatted output, and restoration after a failed write.

Fixes #409.

Validation on Windows with MinGW g++ 13.2, C++17, and the test CMakeLists.txt warning flags (including -Werror):

• Original suite: 250 tests and 905 assertions passed; one existing test skipped.
• Both new regression tests fail against the original header (seven failed assertions).
• Patched full suite: 252 tests and 918 assertions passed; the same existing test skipped.
• git diff --check passed. Uploaded source and test files were compared byte-for-byte with the local files.

AI assistance was used for implementation and review. The reported compiler and test checks were executed locally; other platforms have not been tested locally.