Add integration tests, lib refactor, MIT license, and CI

[Abhishek21g]

Problem

• Closed PR Optimize memory allocations and extract padding calculation helper #1 noted zero functional testing for create/apply roundtrips
• Header block-padding logic is duplicated in create_diff and apply_diff
• Issue Please add an open source license? #3 requests an open-source license
• No CI workflow exists

Fix

• Extract BDiffHeader, format_size, and block_padding_size() into src/lib.rs with unit tests
• Deduplicate padding via shared helper
• Add Linux integration tests (tests/roundtrip.rs) for sparse compactify + base diff roundtrips
• Add GitHub Actions CI (cargo test, cargo clippy -D warnings)
• Add MIT LICENSE (matches sibling CognitionAI OSS repos)

Verification

cargo test --lib          # portable unit tests
cargo test                # full suite on Linux (ext4/xfs + fiemap)
cargo clippy -- -D warnings

Integration tests require Linux (fiemap, copy_file_range).

Fixes #3 (license). Does not duplicate open PR #2 (README blog link).

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