astro(feat[builder]): Emit xref-index.json and objects.inv #449

Job	Run time
qa (3.12)	43s
qa (3.10)	30s
qa (3.11)	35s
qa (3.14)	41s
qa (3.13)	29s
docs	26s
packages	25s
smoke (gp-sphinx)	14s
smoke (sphinx-fonts)	9s
smoke (root-install)	14s
smoke (sphinx-gp-theme)	13s
smoke (sphinx-ux-badges)	16s
smoke (sphinx-autodoc-docutils)	12s
smoke (sphinx-autodoc-sphinx)	16s
smoke (sphinx-autodoc-argparse)	8s
smoke (sphinx-autodoc-api-style)	14s
smoke (sphinx-ux-autodoc-layout)	17s
smoke (sphinx-autodoc-fastmcp)	10s
smoke (sphinx-autodoc-pytest-fixtures)	16s
smoke (sphinx-autodoc-typehints-gp)	17s
	6m 45s

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