feat(02-01): project scaffolding with Cargo, CLI skeleton, and key module

- Initialize Rust project with aes, cbc, hmac, sha2, flate2, clap, rand, anyhow dependencies
- Add clap derive CLI with pack/unpack/inspect subcommands
- Add hardcoded 32-byte AES-256 key constant
- Create stub modules for format, crypto, compression, archive
- All 7 source files in place, cargo build succeeds

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/key.rs

@@ -0,0 +1,9 @@
+/// Hardcoded 32-byte AES-256 key.
+/// Same key is used for AES-256-CBC encryption and HMAC-SHA-256 authentication (v1).
+/// v2 will derive separate subkeys using HKDF.
+pub const KEY: [u8; 32] = [
+    0x7A, 0x35, 0xC1, 0xD9, 0x4F, 0xE8, 0x2B, 0x6A,
+    0x91, 0x0D, 0xF3, 0x58, 0xBC, 0x74, 0xA6, 0x1E,
+    0x42, 0x8F, 0xD0, 0x63, 0xE5, 0x17, 0x9B, 0x2C,
+    0xFA, 0x84, 0x06, 0xCD, 0x3E, 0x79, 0xB5, 0x50,
+];