feat(trade): order book reducer and depth aggregation (#697, #698)

[dev-debbie-umoh]

Summary

Closes #697
Closes #698
Closes #699
Closes #700

Pure, UI-independent modules under apps/web/src/features/trade/lib/orderbook/. No visible behaviour change, so no changelog entry.

• OB-045 book-reducer.ts: snapshot-plus-delta reducer over exact bigint atoms. Revision-contiguity model: buffers deltas during snapshot fetch and replays them, drops those already in the snapshot, ignores duplicate/stale revisions, removes levels on quantity zero, and on any revision hole enters a gap state with actionable: false until a fresh snapshot reconciles.
• OB-046 depth.ts: side-aware grouping (bids down, asks up), cumulative base/quote, spread, crossed/locked detection and visible-depth scales. Grouping is exact integer arithmetic (quantity and quote value conserved, no rounding), never mutates canonical levels, and spread is derived from canonical prices rather than buckets.

Test evidence

vitest run src/features/trade/lib/orderbook: 36 tests pass (reducer replay incl. snapshot handoff, out-of-order/duplicate, quantity-zero removal, gaps; depth incl. one-sided, empty, crossed, regrouping, no mutation). New files pass eslint and prettier and add no tsc errors.

Honest gate report (AGENTS.md)

I did not run a passing full commit gate; main already fails several steps in files this PR does not touch:

• bun lint: 78 existing errors on a clean main (84 with my changes before autofix; my files are lint-clean).
• bun typecheck: 7 errors in useOrderEventPolling.ts.
• bun run check:tokens: 5 violations in existing files.
• bun run test: 9 failures in packages/ui toast.test.tsx.
• test:coverage could not run locally (@vitest/coverage-v8 failed to install; disk was full), so the 90% threshold is unmeasured. build and the indexer integration checks were not run.

Not covered: OB-047/OB-048 (indexer fill data, snapshot/history endpoints) are blocked on unverified upstream depth/fill sources (OB-041/OB-043 still open; see docs/order-book-execution-model.md).

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