Document the T1 item-upsert reconciliation gate

[VolkerChristian]

Outcome

This is the mandatory T1 investigation artifact. It deliberately contains no src/ changes because the brief’s stop gate fired: timeline reconciliation is currently inseparable from the monolithic ConversationWidget::render pass, and the existing compatibility logic does not preserve every unchanged keyed widget for middle insertion/regrouping.

Verified findings

• requiresFullRefresh exists only in Workbench scheduling. It clears exact-content hints; ConversationWidget::render never receives that flag.
• c680e37 failed specifically when a valid exact append and an item upsert shared the 16 ms window. The exact updater returned before topology reconciliation. A lone item upsert still reconciled.
• The previous mixed test used ItemContentReplacedChange, so it could not enter the successful exact-append early-return path.
• A thread-ID-only structural set cannot patch the capped window without recalculating it and cannot identify an activity bucket that gained a row.
• AISuite 61ed370 exposes neither TurnRemovedChange nor ItemRemovedChange. Its supported legacy TurnUpsertedChange can conceal replaceItems=true deletion authority, so moving all turn upserts to an add-only set is unsafe.

The report defines the corrected follow-up scope, full call-site census, and runtime proofs required for the separate render-split implementation.

Validation

• CodexUIFrontendSessionTest: passed
• CodexUIConversationLayoutTest: passed
• One commit; documentation only
• src/ deletion census: 0
• No golden hash or protocol fingerprint changes