Add a plane dialect to myform's translation matrix: bespoke writer+reader pair targeting exact Tiptap-schema conformance for Plane's description_html (not semantic HTML; wire hacks as writer-local post-render steps, typst postformat precedent). Includes the typed checked/task-item core primitive (model/nodes.py). Round-trip property tests against real pulled description_html fixtures, never synthetic. Spec: corpus/scratch/memy-1121-ledger/streams/render-myform-bridge.md 'Staged plan' steps 1-2 — operator 2026-08-08 overrules that stream's case-against ('exactly what I intended myform for'). Migration of corpus call sites (step 3) is a follow-up card.