a function → the full-replace escape hatch, handed the defaults.
a map → merged over the defaults ({ ...defaults, ...map }), so
overriding one component keeps the rest — the phantom-defaults invariant
(§11.2) that stops the MDX missing-reference guard from firing.
Resolve the effective MDX component map from a BootProps.mdxComponents value (MARKDOWN_SYNTAX_SPEC §11.3):
undefined→ the platform DEFAULT_MDX_COMPONENTS (same reference).{ ...defaults, ...map }), so overriding one component keeps the rest — the phantom-defaults invariant (§11.2) that stops the MDX missing-reference guard from firing.