M1 foundation: build/test scaffold + per-method module structure

[hartsock]

Summary

Foundation for the M1 read-side port (docs/ROADMAP.md). Makes gitxtend build,
test, and lint cleanly, and sets up the structure for the per-method sorties.

• Deps pinned: gix 0.70 (full read-side features — status, revision,
  dirwalk, blocking-network-client; the 0.84 tip's status tree requires
  an unpublished gix-worktree-state and won't resolve), pyo3 0.28 (optional),
  tempfile 3.
• PyO3 is opt-in (python feature). cargo test / cargo build run the
  pure-Rust core with no interpreter; maturin enables python +
  extension-module for the wheel.
• Per-method module layout (src/repo/<method>.rs): each M1 method PR adds a
  new file + a 2-line registration in repo/mod.rs + a wrapper in python.rs,
  so the 13 method PRs don't collide.
• error.rs (GitxtendError + Result), a git-CLI temp-dir fixture helper
  for parity tests, and CI + a parity pre-push hook.

Test plan

• cargo test --no-default-features → core tests pass (error + fixtures).
• cargo build --features extension-module → PyO3 layer compiles.
• cargo clippy --no-default-features --all-targets -- -D warnings → clean.
• cargo clippy --features extension-module -- -D warnings → clean.
• cargo fmt --all --check → clean.

Out of scope

• The read methods themselves (R1–R13) + the repo_status roll-up — follow-up
  PRs, each driven by a newt-agent worker with model provenance in the commit
  (Co-Authored-By: <model-id> + Model: trailer).
• The maturin wheel + Python smoke test + version bump → the packaging PR.

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