skylartaylor's review: #1647 and #1671 each introduced native format 4 for a
different capability, so a reader that understood only one could accept a
file carrying the other and silently drop its settings. No release has shipped
v4 (main and v0.12.0 still write at most 3), so v4 is defined as one
capability covering both retained placed-graphic layer options and explicit
all-on PDF layer choices; #1671 is stacked on this branch.
Move the version choice into required_version so every new capability is
decided in one place, document the shared v4 contract on VERSION, and test
that plain, EPS-only and layer-option documents keep 2, 3 and 4 respectively.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
