Adds fusion-react/no-nested-component-definitions, a custom rule in the house
style of the existing detached-spawn guard. A component declared in render is a
new element type every render, so React remounts its subtree on each parent
update and destroys focus, scroll, and local state.
This pattern shipped three times without review or tests catching it: FN-8606's
ModalShell left Planning Mode and Settings untypable, and MailboxModal's
ReplyContextExpandable collapsed expanded reply rows. Tests missed it because
fireEvent.change sets a value without needing the node to stay mounted.
The rule reports PascalCase functions (including memo()/forwardRef()-wrapped)
that return JSX and are declared inside another JSX-returning function.
Lowercase render helpers are deliberately allowed — they are the sanctioned fix.
Escape hatch: // nested-component-allowlist: <reason>.
Scoped to production .tsx, with a vitest guard for the rule itself. Hoists the
two pre-existing violations (ProviderStatusBadge, GitHubStatusBadge in
ModelOnboardingModal) to module scope so the rule lands clean at "error".
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
d193dcdbb0