feat: working clean/smudge round-trip for a Rust subset

[bdelanghe]

What this does

Turns the filter skeleton into a real, git-invoked AST round-trip. With the filter installed, git add stores Rust in canonical form and git checkout returns it — so reformatting never enters history.

git add      →  clean:  your .rs ──Tree-sitter parse──▶ tree ──printer──▶ canonical bytes (stored)
git checkout →  smudge: stored bytes ──▶ working file   (identity; already canonical)

Before this PR the filter was a no-op that didn't even speak git's protocol; the only "round-trip" was a string-prefix marker in a unit test.

Changes

• printer.rs — the AST-native core. Parses Rust with Tree-sitter and re-emits canonical source by walking the tree. Fail-closed: syntax errors reject the commit; any unsupported node kind returns an error rather than silently corrupting code.
• pktline.rs — codec for git's long-running filter pkt-line framing.
• filters.rs — implements the real filter-process protocol (handshake → capabilities → per-blob). clean canonicalizes *.rs, smudge is identity, non-Rust passes through.
• setup.rs / git-ast setup — one command to register the filter + .gitattributes in a repo (idempotent).
• examples/demo.sh — end-to-end proof: a pure reformat produces no diff, a real a + b → a - b change shows a clean one-line diff.

Verified

18 tests pass; cargo fmt --check and cargo clippy --all-targets -- -D warnings are clean. Demonstrated through real git add/git checkout/git diff in a throwaway repo (see examples/demo.sh).

Scope — deliberately honest

• One language, a documented subset of it (functions, params, blocks, let, binary/call/macro expressions, literals, comments). Widening coverage is additive — one arm per node kind.
• Diff and merge drivers remain placeholders. Making those structural depends on the hardest open problem — stable AST node identity across versions — which this PR does not address. Canonical formatting removes formatting churn from history; it does not track a node through a move or rename. See docs/planning/scope.md.

🤖 Generated with Claude Code

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