feat(tui): group nested ACP calls beneath compose

[danielkov]

Summary

Display nested ACP tool calls beneath their owning compose call using explicit parent identities. Each child retains its own status, output, images, and expansion state instead of matching execution to Runlet source nodes.

Motivation

Nested tool activity already arrives over ACP independently of stderr. Use that authoritative lifecycle to keep the transcript useful when source-level progress is unavailable.

Impact

Compose groups show up to 32 child cards per page, with Alt+PageUp/PageDown navigation. Orphans remain visible, late attachment preserves selection, and background descendants continue updating after their parent completes. Duplicate stderr-derived tool presentation is disabled while diagnostic heartbeats remain active.

Technical details

Canonical call records remain independent; indexed parent relationships determine presentation ownership and invalidate the owning transcript cache. Explicit collapse is preserved, and copying grouped output retains each child's logical line identity. Terminal-only content can use parallel raw output when available; terminal stream replay is not added.

Legacy Runlet observation and stderr reducers remain for removal in the follow-up stacked PR.

[kit-code-agent]

Two regressions need attention: filtering transport heartbeats permanently disables retained runtime-status updates after lease expiry, and grouped-call paging consumes the prompt editor's word-navigation shortcuts.

[kit-code-agent]

No issues found. The changes look good to merge.