feat(crypha): scaffold steganography multi-tool with payload engine (M0-M1)
New beginner-tier Go project (crypha): a multi-format steganography tool that hides an encrypted payload across five carriers (image, audio, QR, zero-width text, PDF) from a cobra CLI and a planned bubbletea TUI.
M0 scaffold: cobra skeleton, config constants, justfile, golangci config. M1 engine spine: internal/bitio (MSB-first bit reader/writer), internal/payload (versioned AEAD envelope: Argon2id KDF, ChaCha20-Poly1305 default / AES-256-GCM alternate, flate compression, CRC32 integrity, header-as-AAD, hostile-parameter hardening), internal/carrier (Carrier interface + self-registering registry).
Verified: go build, go vet, gofmt, golangci-lint (0 issues), go test -race all green; coverage bitio 100% / carrier 91% / payload 88%.
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