First slice of the Steganography Multi-Tool. Sets up the project structure (src layout, pyproject, justfile) following the repo conventions and implements the first technique: zero-width text, which hides bytes inside a visible carrier using invisible Unicode characters with a 32-bit length prefix.
Includes a Typer CLI with hide/reveal subcommands, UTF-8-safe file and stdout I/O, and property-based round-trip tests with hypothesis. ruff and mypy strict are clean.
Part of the phased approach agreed in the issue; audio/QR and PDF/git techniques follow in later PRs.
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