Stop re-rendering the Workspace shell on every streaming delta

[juacker]

Problem

Streaming text deltas arrive many times per second, and the accumulator lived inside SessionState — so each appendDelta gave sessions[sessionId] a new identity (immer structural sharing), re-rendering every whole-session subscriber per token chunk. The worst offender: the Workspace page (2,200+ lines of component body) subscribed to the entire session object at Workspace.tsx:1536. On WebKitGTK this is measurable render pressure (planet9 profiling: iowait there is GPU-fence accounting, i.e. compositor/frame pressure — exactly what per-token full-page re-renders feed).

Based on external review feedback about the store's zustand usage — validated against the code before implementing.

Changes

1. Streaming accumulator moved to a top-level streamingText map in the same store (sessionId → messageId → text).

• Deltas now invalidate only streamingText subscribers; sessions[sessionId] identity is untouched (pinned by a new identity-stability test).
• appendDelta guards its isStreaming write so repeat deltas leave the session draft unmodified.
• Deliberately not a separate store: completeMessage still swaps accumulator-for-final-message atomically in one set() — a second store would open a one-frame flicker window between "streamed text gone" and "final message present".
• All touchpoints updated: completeMessage, updateMessageContent, removeMessage, setRunStatus (terminal clears), removeSession (no leak), loadSessionData (snapshot refresh can no longer wipe in-flight text by construction).

2. Narrow selectors in the page shell.

• Workspace subscribes per-field (messages, toolCalls, isStreaming, runStartedAt, queued ids, pagination flags, runs); the per-delta streamingText subscription moves down into ChatFirstLayout, so token chunks re-render only the chat area.
• Same treatment for WorkspaceTaskTranscriptPanel and useAssistantSession.
• Stable EMPTY_* fallback constants (outside selectors — selector results stay referentially stable for useSyncExternalStore).

Behavior

No user-visible behavior change intended: event ingestion is unaffected (selectors gate re-renders, not data), fallback semantics (?? vs ||) preserved exactly, ChatMessageList's public prop API unchanged.

Validation

• sessionStore tests: 29 (5 new, incl. the perf contract: session identity unchanged across deltas), useAssistantEvents: 4 — all green.
• Full suite matches main's baseline exactly; tsc --noEmit and eslint clean.
• Independent full-scope review: production_quality, no findings.

Draft for @juacker to test live streaming, run completion (no text flicker at the streamed→final swap), workspace switching, and the task transcript panel.