T1a: Preserve keyed timeline segments during reconciliation

[VolkerChristian]

Reproduction gate

The test was written and run before the fix against merged master 3679e38.

• Live activity-bucket growth retained its segment ID and passed as the control.
• Inserting a new segment between two existing segments reproduced the defect: both unchanged-widget identity and final layout order failed.

Change

Replace the prefix-alignment/whole-turn teardown with an exhaustive keyed reconciliation over the desired segment order:

• remove and destroy only genuinely absent segment widgets;
• move surviving widgets to their desired layout indices without reparenting or rebuilding them;
• create only genuinely new or presentation-incompatible widgets;
• purge activity-row lookup state synchronously when its segment disappears;
• preserve the pending viewport anchor across survivor moves and unrelated removal, clearing it only when its widget is destroyed.

Turn-level compatibility, render() call structure, and the exact-content fast path are unchanged.

Runtime proof

CodexUIConversationLayoutTest now covers:

• a live activity bucket gaining a row while retaining its QWidget*;
• middle insertion with identity preservation and correct order;
• arbitrary regroup/reorder with all 20 unchanged keys retaining their original QWidget*;
• genuine removal leaving the keyed lookup immediately and becoming a null QPointer after deferred deletion;
• anchor survival across reorder and unrelated removal, plus clearing on destruction of the anchor widget.

Validation

• Canonical incremental build root: /home/voc/projects/drafts/.codex-builds/codexui-final-verify-20260822
• Focused CodexUIConversationLayoutTest: passed
• Full CodexUI suite: 7/7 passed
• Source deletion census: 86 removed lines, all from replacing the prefix/whole-layout teardown and its superseded positioning calls.
• No golden hash or protocol fingerprint changed.