feat(02-01): format types, crypto pipeline, and compression module

- Implement Header and TocEntry structs matching FORMAT.md byte layout
- Add write_header (40 bytes) and write_toc_entry (101+name_len bytes) serialization
- Add read_header, read_toc_entry, read_toc deserialization with validation
- Implement AES-256-CBC encrypt/decrypt with PKCS7 padding via cbc crate
- Add HMAC-SHA-256 compute/verify over IV||ciphertext (encrypt-then-MAC)
- Add SHA-256 hash for original file integrity
- Implement gzip compress/decompress with deterministic mtime(0)
- Add should_compress heuristic for known compressed file extensions

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

src/compression.rs

@@ -1,2 +1,51 @@
-// Gzip compression/decompression and compression heuristic.
-// Will be implemented in Task 2.
+use flate2::read::GzDecoder;
+use flate2::{Compression, GzBuilder};
+use std::io::{Read, Write};
+
+/// Gzip-compress data with reproducible output (mtime zeroed).
+///
+/// Uses `GzBuilder::new().mtime(0)` to zero the gzip timestamp,
+/// ensuring reproducible compressed output for testing.
+pub fn compress(data: &[u8]) -> anyhow::Result<Vec<u8>> {
+    let mut encoder = GzBuilder::new()
+        .mtime(0)
+        .write(Vec::new(), Compression::default());
+    encoder.write_all(data)?;
+    let compressed = encoder.finish()?;
+    Ok(compressed)
+}
+
+/// Gzip-decompress data.
+pub fn decompress(data: &[u8]) -> anyhow::Result<Vec<u8>> {
+    let mut decoder = GzDecoder::new(data);
+    let mut decompressed = Vec::new();
+    decoder.read_to_end(&mut decompressed)?;
+    Ok(decompressed)
+}
+
+/// Determine if a file should be compressed based on filename and exclusion list.
+///
+/// Returns false for:
+/// - Files matching any entry in `no_compress_list` (by suffix or exact match)
+/// - Files with known compressed extensions (apk, zip, gz, etc.)
+///
+/// Returns true otherwise.
+pub fn should_compress(filename: &str, no_compress_list: &[String]) -> bool {
+    // Check explicit exclusion list
+    if no_compress_list
+        .iter()
+        .any(|nc| filename.ends_with(nc) || filename == nc)
+    {
+        return false;
+    }
+
+    // Check known compressed extensions
+    let ext = filename.rsplit('.').next().unwrap_or("").to_lowercase();
+    !matches!(
+        ext.as_str(),
+        "apk" | "zip" | "gz" | "bz2" | "xz" | "zst"
+            | "png" | "jpg" | "jpeg" | "gif" | "webp"
+            | "mp4" | "mp3" | "aac" | "ogg" | "flac"
+            | "7z" | "rar" | "jar"
+    )
+}