feat(pack): add PackStats and stats_pack for pack file object statistics

[sunruizhesrz]

📝 Background

git-internal can fully decode pack files (Pack::decode), but lacks a lightweight way to get object type counts (commits, trees, blobs, tags, deltas) without full decompression and delta resolution.

This PR adds a read‑only statistics interface that reuses existing parsing logic.

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✨ Changes

📦 New PackStats struct (in decode.rs)

pub struct PackStats {
    pub total:   usize,
    pub commits: usize,
    pub trees:   usize,
    pub blobs:   usize,
    pub tags:    usize,
    pub deltas:  usize,
}

⚙️ New Pack::stats_pack method

pub fn stats_pack(path: PathBuf) -> Result<PackStats, GitError>

Step	Description
1️⃣	Validate pack header, read total object count
2️⃣	Stream objects via decode_pack_object
3️⃣	Classify by type (deltas normalized → deltas)
4️⃣	Return PackStats or propagate GitError

🧪 Tests (4)

Test	Scenario
test_stats_pack_small_sha1	19 objects, basic counting ✅
test_stats_pack_medium_sha1_has_deltas	35k objects with deltas ✅
test_stats_pack_file_not_found	Missing file error ❌
test_stats_pack_invalid_pack_magic	Corrupt header error ❌

✅ All tests pass · cargo fmt · clippy clean

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✅ Verification

cargo test --lib internal::pack::decode::

let stats = Pack::stats_pack("tests/data/packs/small-sha1.pack".into())?;
// Output: PackStats { total: 19, commits: 1, trees: 4, blobs: 14, tags: 0, deltas: 0 }

---

🧠 Design

Principle	Implementation
Reuse	Uses decode_pack_object → consistency, minimal duplication
Lightweight	No delta reconstruction, no decompression, no cache writes
Error handling	All errors via ? → no panics
Invariant	commits + trees + blobs + tags + deltas == total (tested)

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📂 Files changed

• src/internal/pack/decode.rs (only)

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📎 Compatibility

• ✅ Fully backward compatible – adds new API only
• ✅ No new dependencies

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Thanks for reviewing! 🙏

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