Virtualize the native Qt conversation view

[VolkerChristian]

Summary

• replace the conversation-wide QWidget/layout hierarchy with a thin NodeGraph-backed QAbstractListModel and a narrowly customized QAbstractItemView
• preserve stable NodeRef targeting and emit precise insert, remove, move, and row-local data-change notifications
• use a logarithmic variable-height index, exact pixel anchoring, atomic structural staging, and bounded direct-tail insertion
• delegate-paint passive rows while retaining real viewport-bounded widgets for selection, Markdown interaction, links, images, command output, folding, focus, and other rich controls
• preserve the existing two-thread, shared-NodeGraph, typed-SPSC/eventfd, and direct CodexBridge boundaries
• include narrow follow-up fixes for recursive outside-text mouse forwarding and first-child active-turn border growth

Performance

The production Debug benchmark runs under Xvfb/xcb in a 900 x 700 viewport with 240 scroll positions.

Rows	Old reveal	New reveal	Old descendants	New descendants	Old sweep	New sweep
320	733 ms	16 ms	4,209	8	329.7 ms	260.2 ms
1,280	4,040 ms	25 ms	16,809	8	325.2 ms	278.1 ms
10,000	not safely run	97 ms	about 131,000 extrapolated	8	not run	355.9 ms

At 1,280 rows, initial reveal is approximately 162 times faster and peak resident memory is approximately 70 percent lower. Ten thousand passive rows create zero ConversationCard widgets. Bounded tail append takes 1.44 to 1.73 ms across 320 to 10,000 rows with zero model-index and section-range rebuilds.

A fresh post-fix spot check measured 7/16/96 ms reveal and 0.320/0.318/0.314 ms tail append at 320/1,280/10,000 rows.

Validation

• persistent incremental Debug build succeeds
• latest native suite passes 19/19
• ASan/UBSan migration suite passes 19/19
• supported NodeGraph/queue/worker TSan suite passes 5/5
• WebUI release qualification passes 83/83 plus profile, production build, browser checks, and relocatable artifact verification
• deterministic 10,000-row model/view, anchoring, offscreen-work, interaction, and event-loop regressions pass
• full application exercised against an isolated persistent CodexBridge/app-server under Xvfb
• recordings cover long selection, paging, scrolling during streaming and command completion, steering, approvals, input requests, selection/copy, folding, focus, and direct-tail arrival
• git diff --check passes

Detailed architecture, ownership, behavior matrix, measurements, and recording inventory are in docs/qt-virtualized-conversation-view.md.