fix(chat): scope a comparison pane's attachments to its own lane

[garfiec]

Summary

A Compare Models turn rendered the message's attachment-driven surfaces twice, once per pane. Each pane filters content per lane but carried attachments through whole, so anything a bubble draws from the attachment list rather than from a pane-filtered part was emitted in both panes. Panes now see only the attachments their own lane produced.

Three duplications this removes:

• An inline set_memory in one lane rendered its own card in that pane and — because its tool call is absent from the other pane's content — read as an orphan there and got a second card.
• A background-memory-agent write matches no call in either pane, so both rendered it.
• Office-doc previews render from message.attachments with no tool-call join at all, and duplicated unconditionally.

Closes #359.

Changes

• attachmentsForPane (util/ComparisonDisplay.kt) — an attachment follows the lane whose parts contain the call that produced it; one attributable to no call in the turn renders once, in the primary pane.
• Wired into buildComparisonDisplayMessages' parallel branch and into collapseParallelToPrimary. In the single list this is a removal, not only a de-duplication: the added agent's attachments are dropped alongside its content, so a secondary-lane office preview or memory write that previously rendered there now renders nowhere. That path also feeds in-conversation search and the iOS screen.
• Where a pane ends up with no attributed parts — the Final→reload gap, or the in-branch empty-pane fallback — there is nothing to scope against, so the turn's attachments go to the primary pane, which is where they land once the attributed message arrives.
• renderedToolCallIds gains a KDoc note on why it stops one level down and is not interchangeable with the recursive walk.
• Module doc records the invariant.

Testing

• ./gradlew test detektMetadataCommonMain detekt :app:lint :app:assembleDebug — green.
• 7 new ComparisonDisplayTest cases. 6 of the 7 fail against the pre-change file; the seventh is the non-parallel regression guard and passes either way.
• Device A/B on an emulator against a seeded comparison turn carrying three memory writes — one per lane plus one attributable to neither. Before: every write rendered twice. After: each renders once, in its own lane, with the unattributable one in the primary pane. The single-list path was checked by toggling Compare Models off. No crashes; one pre-existing empty-body decode error appears identically on both builds.

Notes

• Upstream hands MemoryArtifacts every attachment once above both columns, with no toolCallId filter (ParallelContent.tsx:275, MemoryArtifacts.tsx). Placing an unattributable write in the primary pane matches that. Routing an attributable one to its own lane is a deliberate divergence, needed because mobile also renders an inline per-call card that upstream has no equivalent of.
• Scoping lives in ComparisonDisplay rather than at the render site because the panes are derived in a pure function, so the fix is assertable in JVM tests — feature:chat has only an on-device Compose harness, the same reason sendButtonModeFor is a pure transform pulled out of its composable. A render-site fix would have to be repeated on every bubble surface that reads message.attachments.
• Ownership uses outputToolCallIds (recursive) and not the similarly-named renderedToolCallIds, which stops one level down because that is where the subagent trace recursion stops — a call two levels down is drawn by nothing. outputToolCallIds now serves two purposes across three call sites: attachment hoisting (activity groups and subagent cards) and this lane attribution.
• The background-agent case was exercised with a seeded attachment of that shape rather than a live background memory agent (memory.agent.enabled is not configured on the test server), so the rendering path is covered but not the server behaviour that produces it.
• The office-preview duplicate follows from the same rule but was not observed on a device.

[github-actions]

Android debug APK

Artifact: switchboard-android-debug-363
Download: switchboard-android-debug-363.zip
Retention: 90 days
Commit: b2136c2fc0d49abc309bc54c1d05a6502cf0a442

Download requires a GitHub login. Installs over previous debug builds without uninstalling (stable signing key).