B2 follow-up: parent-scoped conditional rules in inline grids ("paid invoice → lock lines") #1581

Context

B2 field conditional rules (visibleWhen/readonlyWhen/requiredWhen) ship for standard form fields (PR #1578) and row-scoped in inline grids (PR #1579). The parent-scoped case — a line-item cell that reacts to the header record (e.g. lock all lines once parent.status == 'paid') — is deferred.

Why it's deferred (root cause)

The grid and the header ObjectForm both live inside MasterDetailForm. To re-gate grid cells when a header field (status) changes, the grid must re-render. But the watcher that scrapes the live header record into state re-renders MasterDetailForm → re-renders ObjectForm → fires its form.reset, wiping the user's input mid-edit/submit (the historical "Create, nothing happens" regression). Verified: broadening the watcher reproduced the wipe; backing it out restored green.

Plan

• Extract the line-items section of packages/plugin-form/src/MasterDetailForm.tsx into a child component that owns the parentRecord state + the form-host watcher.
• A header change then re-renders only the grid subtree, never the reset-sensitive header form.
• Dedupe the scraped record (only setState on real value changes) to avoid mid-submit churn.

Already in place (plumbing)

• @object-ui/core: evalFieldPredicate(pred, record, fallback, previous?, scope?) and resolveFieldRuleState(..., scope?) accept an extra scope (binds parent.*) — unit-tested.
• @object-ui/fields: GridField accepts a contextRecord prop (bound as parent); deriveMasterDetail already carries readonlyWhen/requiredWhen onto columns.

So this is wiring + render-isolation, not a redesign.

Acceptance

• Showcase invoice_line re-adds readonlyWhen: "parent.status == 'paid'" on quantity/unit_price/product (framework example).
• Live e2e: New Invoice → set Status = Paid → line cells lock; back to Draft → unlock.
• Full live e2e suite stays green (no form.reset wipe regression on form-view-subforms / master-detail).

See docs/adr/0036-field-conditional-rules.md (the "Inline grids" + deferred-parent note).