fix(git): kill upload-pack when the client disconnects

[Temikus]

What kind of change does this PR introduce?

• fix
• feat
• refactor
• perf
• docs
• test
• build / ci
• chore

Summary
Point 3 follow-up from #45. When a client hangs up mid-serve, upload-pack and its pack-objects child kept running. pack-objects writes nothing while it counts and compresses, so it never gets SIGPIPE and can pack for minutes with nobody reading.

upload-pack now runs in its own process group. TimedReader signals a oneshot at EOF; if the stream is dropped before that, the reaper SIGKILLs the group before reaping, so the group ID cannot be recycled. The reaper is spawned before the request write, so a request dropped mid-write is cleaned up too.

rustix (unix only) is a new direct dependency for a safe killpg. It was already in the lockfile via tempfile.

Tests

• Added / updated tests (unit, plus integration where it fits)

The e2e test uses a fake git_binary whose upload-pack starts a silent grandchild, then drops the TCP connection and checks the grandchild dies. It fails on main and passes here.

Checklist

• CI is green locally: cargo fmt --all --check, cargo clippy --all-targets --all-features --locked -- -D warnings, cargo test --all-features
• Commits follow Conventional Commits; AI-assisted commits carry an Assisted-by: trailer (see CONTRIBUTING.md / AGENTS.md)
• Preserves the read-only, pull-only invariant (no push or proactive replication to upstream)
• Docs / README updated if behavior or flags changed

Breaking change?
No.

[codecov]

Codecov Report

❌ Patch coverage is 98.94737% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 96.93%. Comparing base (66a0417) to head (facb844).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/git.rs	98.94%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #47      +/-   ##
==========================================
+ Coverage   96.80%   96.93%   +0.12%     
==========================================
  Files           6        6              
  Lines        2065     2151      +86     
==========================================
+ Hits         1999     2085      +86     
  Misses         66       66              

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