OB-076: Gate reduce-only and trigger controls by execution support #729

Priority: P0 · Size: M · Area: order-entry · Depends on: OB-003, OB-004, OB-043, OB-071

Scope: Wire reduce-only and take-profit/stop-loss only when the gateway
enforces their semantics. Resolve current sidecar placeholders so controls cannot
appear successful without creating the promised orders.

Acceptance criteria:

• Supported options round-trip through simulation/submission fixtures and invalid combinations are rejected before signing.
• If attached orders require separate transactions, their independent outcomes are explicit; unsupported controls cannot be enabled through stored preferences.

Season context

Build a standard order book trading experience for SO4 on Stellar: a compact watchlist and market header, a large chart, an adjacent order book/recent-trades panel, a right-hand order ticket, and account tables below. Preserve SO4's brand and shared design tokens. Keep verified perpetuals capabilities; do not add Pro mode, EVM wallet connection, new margin modes, TWAP, or spot/perps transfers solely to imitate the reference layout.

An order book layout does not establish an executable order book. Matching, resting depth, partial fills, signing, and settlement need verified sources; account orders, oracle prices, pool liquidity, and illustrative data are not substitutes for executable depth. Preserve geometry during loading and live updates. Motion should clarify state changes, with reduced-motion, focus, touch, and interruption behavior included.

Extend the existing custom @workspace/ui/components/toast implementation; do not install Sonner or a second toaster. Use TanStack Query for scoped caching, targeted refresh, and explicit indexer catch-up after confirmed transactions.

Common completion rules

• Follow AGENTS.md, including reproduction and regression
  coverage for bug fixes, the full ordered commit gate, and integration checks for
  web/indexer changes. Do not weaken checks. Dependency/build changes require the
  clean-room verification specified there.
• Use existing components and named design tokens from
  packages/ui. Token changes include the corresponding
  DESIGN.md documentation. No parallel theme or toast system.
• Use explicit MSW handlers for network tests. Include relevant success, empty,
  stale, and failure behavior; keep timers and streams deterministic. Visual work
  includes desktop/mobile and reduced-motion review, with relevant theme coverage.
• Include a user-visible changelog entry when required by the repository's
  entry format. Documentation changes
  follow apps/docs/CONTRIBUTING.md.
• Distinguish wallet approval, submission, ledger confirmation, order acceptance,
  execution/partial fill, and indexer catch-up. A transaction confirmation must
  never falsely imply that a resting order has filled.
• Use targeted invalidation after confirmed changes, and explicitly handle
  inactive queries when their views next open. Invalidating an active query marks
  it stale and triggers background refetch; it does not prove an indexer has
  caught up. See TanStack Query invalidation.
• Cache freshness and retention are separate choices. Define them by data class,
  rather than treating a long staleTime as a real-time strategy. See
  TanStack Query defaults.
• Measure speed on the baseline device/network profile in OB-010. Initial field
  targets are p75 LCP ≤ 2.5 s, INP ≤ 200 ms, and CLS ≤ 0.1, reported separately
  for landing and trading and for mobile/desktop. Lab results are diagnostics,
  not a substitute for field percentiles. See Web Vitals.

Priority key: P0 = prerequisite/correctness; P1 = core experience; P2 = later polish. Size: S/M/L describes relative scope, not a delivery-time promise.