Refactor web stores into atomic slices ready to split ChatView

[justsomelegs]

What Changed

Refactored the web orchestration store from broad array-based state into a more atomic normalized shape.

Main changes:

• replaces projects[] / threads[] as the primary store shape with keyed records and id lists
• splits thread state into narrower slices instead of storing one large thread object as the main reactive unit
• updates web consumers to read direct keyed state instead of subscribing to large collections and scanning them
• updates the web test suite and fixtures to use the new normalized store model

Old Pattern

const threads = useStore((s) => s.threads);
const activeThread = threads.find((thread) => thread.id === threadId);
const messages = activeThread?.messages ?? [];

const project = useStore((s) =>
  s.projects.find((project) => project.id === projectId)
);

New Pattern

const shell = useStore((s) => s.threadShellById[threadId]);
const session = useStore((s) => s.threadSessionById[threadId] ?? null);
const messageIds = useStore((s) => s.messageIdsByThreadId[threadId] ?? EMPTY_IDS);

const project = useStore((s) => s.projectById[projectId]);

Why

The old store shape forced many consumers to read broad collections and then search inside them. That made fine-grained subscriptions hard and set us up for unnecessary rerenders once we start breaking large components into smaller ones.

This change establishes the state model we actually want to build on:

• direct keyed access for exact reads
• smaller reactive boundaries inside thread state
• a cleaner base for future work on Sidebar, ChatView, and other hot UI paths

Keeping this as a store-shape-first PR also keeps the follow-up performance work simpler. Future PRs can focus on splitting components and wiring each subcomponent to only the exact slice it needs.

Checklist

• This PR is small and focused
• I explained what changed and why

Note

Refactor web store state from arrays to normalized entity maps for projects and threads

• Replaces projects: Project[] and threads: Thread[] in AppState with normalized maps (projectById, threadShellById, threadSessionById, threadTurnStateById) and id arrays, splitting thread data into immutable shell and volatile turn state slices.
• Adds selectProjects, selectThreads, createThreadSelector, createProjectSelector, and createSidebarThreadSummarySelector to derive denormalized objects from the new normalized state; createThreadSelector memoizes by reference equality to avoid unnecessary re-renders.
• Updates all components (ChatView, BranchToolbar, DiffPanel, Sidebar, GitActionsControl, etc.) and hooks to use selectors or direct map lookups instead of array scans.
• Rewrites syncServerReadModel and applyOrchestrationEvent in store.ts to write into normalized slices via new writeThreadState/removeThreadState/buildThreadState helpers.
• Risk: any code directly reading the removed state.projects, state.threads, or state.sidebarThreadsById fields will break; all in-tree callers have been migrated to selectors.

^{Macroscope summarized 025d832.}

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[macroscopeapp]

Approvability

Verdict: Needs human review

Major refactor of web store architecture from array-based to normalized/atomic slices, restructuring core state management infrastructure across multiple files. The scope and criticality of these internal data structure changes warrant human review to ensure the migration is complete and correct.

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[justsomelegs]

fixed CI issues, should now pass :)

[juliusmarminge]

Got #1765 stacked on top of this so will merge it as a stack when that one's ready!