Preserve parent unknown arguments across subcommands

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parse_known_args discards unknown arguments collected by a parent parser when it dispatches to a subcommand. With nested subcommands, only the deepest command's unknown arguments survive.

For example, tool --parent-extra P run --child-extra C leaf --leaf-extra L currently returns only --leaf-extra and L. This appends each child's unknown arguments to its parent's collected arguments, preserving all six tokens in order.

Adds coverage for parent-only and child-only unknowns, ordering and duplicates around known values, compound options, nested commands, argc/argv, propagated conversion errors, and unchanged strict-parser rejection.

Validation on upstream dc6bd1e with Windows UCRT GCC 13.2 and C++17:

• Original reproduction: 2/6 tokens preserved, exit 1.
• New regression tests on the original header: 2 test cases fail.
• Patched full suite: 267 passed, 954 assertions, 1 existing skipped test, using the repository warning flags with -Werror.
• Patched reproduction: 6/6 tokens preserved, exit 0; git diff --check passed.

This is independent of #436 and does not change argument validation or preprocessing. AI-assisted implementation and review; the patch was inspected and the reported tests were run locally.