Build native core during contributor setup

[Alek99]

Summary

• build the required release native core as part of make setup
• route the contributor quick starts through make setup and document its Rust requirement
• add a regression test covering the setup recipe and contributor docs

Root cause

The contributor quick start only created a virtual environment and installed the project editable. An editable install can complete without leaving libxy_core in one of the development loader paths, so the documented make check step then fails while importing xy.kernels.

make setup now runs cargo build --release explicitly, ensuring a source checkout is ready for the documented fast gate.

Closes #77.

Validation

• clean setup with XY_SKIP_CARGO=1 forced the editable-install path to omit the core; the explicit setup build then produced target/release/libxy_core.dylib and xy.kernels.BACKEND == "native"
• .venv/bin/python -m pytest -q tests/test_verify_local.py — 62 passed
• make check-docs — claim guardrails passed and 68 tests passed
• git diff --check

The broader make check run reached 1,843 passed and 66 skipped tests, with one unrelated local Chromium alignment failure in test_midpoint_annotation_labels_are_visually_centered; this patch does not change chart rendering or browser assets.

[codspeed-hq]

Merging this PR will not alter performance

✅ 94 untouched benchmarks
⏩ 1 skipped benchmark¹

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_{Comparing agent/build-native-core-in-setup (4e21fa8) with main (cfb5d12)}

Footnotes

1. 1 benchmark was skipped, so the baseline result was used instead. If it was deleted from the codebase, click here and archive it to remove it from the performance reports. ↩