Hoshe's review finding: the build_step_canvas row-space convention block
still described region_centre_latitude_deg as pole-anchored/bugged and
said 'T-1186 stays open' — both false after this PR's own first commit.
Rewritten to reflect the fixed state; the deliberate signed-convention
rationale and the layer_proxy deferral (still true, still T-1181's scope)
are preserved.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>