chore(callcenter): gitignore zone-poison test-fixture build artifacts

`cargo test -p lance-graph-callcenter` compiles the zone-poison-fixtures
test crate, generating a Cargo.lock + target/ inside it. The fixture's
sources are tracked but its lock is not (no fixture lock is tracked in the
repo; the workspace root Cargo.lock is the tracked one, so a blanket root
ignore is not an option). A fixture-scoped .gitignore keeps the tree clean.

Author: claude

crates/lance-graph-callcenter/tests/zone-poison-fixtures/.gitignore

@@ -0,0 +1,8 @@
+# Build artifacts generated when this test-fixture crate compiles during
+# `cargo test -p lance-graph-callcenter`. The fixture's sources (Cargo.toml,
+# build.rs, src/) are tracked; its lock + target are not (no fixture lock is
+# tracked anywhere in the repo, and the workspace root Cargo.lock is the
+# tracked one). A fixture-scoped ignore keeps the tree clean without
+# un-tracking the workspace lock at root.
+Cargo.lock
+/target/