perf(vscode): optimize large session load times and eliminate reactive cascades

[marius-kilocode]

Opening large sessions (500+ messages and thousands of parts) previously caused main-thread hitches and latency spikes during first-time loading in the webview.

Root Causes

1. Cascading Reactive Subscriptions on Part Hydration: getParts(messageID) accessed store.parts[messageID] inside layout memos (turns, rows, partitionRows, railEntries), subscribing every message in the session to store.parts. When virtualized rows hydrated parts on mount, mutating store.parts invalidated all memoized rows across the entire transcript, triggering recomputation cascades for every mounted row.
2. Recursive Tool Store Proxy Churn: rebuildToolParts always passed the full tool list through reconcile(tools, { key: "id" }) on cold replace loads, recursively traversing and creating reactive proxies for hundreds of tool calls that were not visible on screen.
3. Uncapped Subagent Task Output IPC Payloads: Historical task tool outputs were not routed through slimmers, resulting in multi-megabyte payloads being serialized and passed across the extension IPC bridge.
4. Redundant Hierarchy Scans on Idle Opens: sessionIDs, visibleToolParts, and activeUserMessageID ran full message scans and allocated sets during idle session opens even when sessions had zero subagent tasks.

Changes

• Wrapped store.parts[messageID] reads in untrack() and prioritized stash.peek(), allowing layout memos to read parts without establishing reactive dependencies that get invalidated during part hydration.
• Passed mode to rebuildToolParts / setTools and assigned tools directly on replace loads, bypassing recursive store reconciliation.
• Added task: slimOutput to metadata slimmers to cap historical task outputs over the IPC bridge.
• Added fast-path early returns in visibleToolParts, sessionIDs, and activeUserMessageID for idle sessions with no task tools.

Performance Profile & Benchmark

First-time cold load of session wt-1786000915486-13 (544 messages, 2,800+ parts) in the VS Code webview:

Metric	Baseline	Optimized	Improvement
Script Execution Duration	836.75 ms	602.07 ms	-28.0% (-234.68 ms)
Layout Duration	121.76 ms	85.30 ms	-29.9%
Longest Main Thread Task	143.70 ms	104.42 ms	-27.3%
CPU Profile Nodes	48,194	25,279	-47.5%
Transient DOM Nodes	6,921	3,485	-49.6%
Layout Objects Created	2,698	1,242	-54.0%
JS Event Listeners	1,940	746	-61.5%