v0.21.0

Attach options can now be declared per catalog, so a worker serving several catalogs can gate one of them without gating the rest.

• WithAttachOptionsForCatalog(catalog, specs...) registers option specs for a single alias catalog (see WithCatalogAliasInfo). They are advertised on that catalog's vgi_catalogs() row and enforced only when it is the ATTACH target; the primary catalog keeps using WithAttachOptions.
• The example attach-options worker now also serves attach_options_required, a catalog gated on an option the caller must supply — the fixture behind the integration suite's attach/attach_options_required.test.
• MissingAttachOptionsError now single-quotes option names, matching the Python and Java implementations (it used Go's %q, so the shared test's expected text never matched).
• Examples self-kill portably, so the package builds on Windows.