T6 — Lock down viewport and control-generation invariants

[VolkerChristian]

Summary

• protect off-bottom presentation freeze across repeated canonical updates
• verify rapid tail-follow updates remain pinned while retaining stable widgets
• verify reconciliation preserves a surviving anchor's observable viewport position and existing anchor-removal boundaries
• retain the complete upcoming-turn control tree across harmless refreshes
• prevent obsolete turn-bound controls from submitting through click or Ctrl+Enter

Tests only; no production, AISuite, protocol, paging, or budget changes.

Validation

• full incremental build in /home/voc/projects/drafts/CodexUI/build-codex
• ctest --test-dir /home/voc/projects/drafts/CodexUI/build-codex --output-on-failure
• 7/7 tests passed
• git diff --check

Audit

• ConversationWidget::render has one production call site; unchanged
• action-state forwarding call sites are unchanged
• deletion census: no teardown, deletion, or widget-removal paths changed