feat(drips): flag-gated slices for spikes 655/656/657/660 (#517 #518 #520 #523)

[dimka90]

Summary

Four flag-gated partial slices for the assigned extreme spikes, following the existing phase-154 convention (documented in PROJECT_ARCHITECTURE.md §10). Each slice is self-contained lib/ logic with unit tests, a new feature flag (default off), and a rollback row. No production route wiring is added, so behavior is unchanged until a flag is enabled.

• lib/soroban-read-coalescer.ts + flag phase-158
• lib/owner-batch-loader.ts + flag phase-159
• lib/web-vitals-budgets.ts + flag phase-160
• lib/phase-error-taxonomy.ts + flag phase-161

Issues

Issue 517: Spike 655 — Soroban RPC request batching plus dedupe for simulation storms

Problem: Repeated/identical simulations (same get_config/balance/owner_of within a request) each hit simulateTransaction, so duplicate reads and 429/5xx retry storms multiply RPC load.
Resolution: lib/soroban-read-coalescer.ts — process-local coalescer that shares one in-flight promise for identical (contractId, method, args) reads and memoizes results for a short TTL, with optional stale-on-error fallback and closed-label metrics (lib/__tests__/soroban-read-coalescer.test.ts). Flag phase-158. Follow-ups: JSON-RPC array batching / cross-process cache.
Closes #517

Issue 518: Spike 656 — N+1 elimination in explore, world and profile reads with DataLoader batching

Problem: Explore/world/profile issue one owner_of(id) simulation per token (N+1), multiplying round-trips on every listing page.
Resolution: lib/owner-batch-loader.ts — a DataLoader-style per-tick coalescing loader with key dedupe, maxBatchSize chunking, cross-tick memoization, and per-key error results (lib/__tests__/owner-batch-loader.test.ts). Flag phase-159. Follow-ups: route wiring, index pushdown, sidecar mget.
Closes #518

Issue 520: Spike 657 — Web Vitals budgets with CI gates plus RUM pipeline

Problem: No per-route performance budget exists, so route regressions go unnoticed; the app only ships a page-view beacon.
Resolution: lib/web-vitals-budgets.ts — per-route LCP/INP/CLS budget table with dynamic-segment matching, evaluateVital good/needs-improvement/poor rating, and summarizeVitals worst-per-vital pass/fail summary (lib/__tests__/web-vitals-budgets.test.ts). Flag phase-160. Follow-ups: live collector, RUM transport, PR-blocking gate.
Closes #520

Issue 523: Spike 660 — Public TypeScript SDK for the PHASE protocol with error taxonomy

Problem: The SDK-facing error surface is free-form strings (parsePhaseContractErrorCode + humanizePhaseHostErrorMessage), so consumers cannot branch on a stable enum.
Resolution: lib/phase-error-taxonomy.ts — a code-stable PhaseError taxonomy mirroring contract discriminants 1..39 with retryable hints and userMessageKeys, plus mapPhaseContractError that converts a raw host error into a typed descriptor (lib/__tests__/phase-error-taxonomy.test.ts). Flag phase-161. Follow-ups: Horizon/RPC transport taxonomy, retry/circuit policy, dual CJS/ESM packaging.
Closes #523

Verification

• npx vitest run lib/__tests__/{soroban-read-coalescer,owner-batch-loader,web-vitals-budgets,phase-error-taxonomy}.test.ts → 26 passed / 26.
• npx tsc --noEmit → no errors in touched files (pre-existing errors elsewhere on main).
• Full npx vitest run still has unrelated pre-existing failures (phase-142/143/146/147 AI suites, artist-profile-client import); none touch this change.

Flags / rollback

All four flags default off. Rollback = unset NEXT_PUBLIC_FEATURE_PHASE_158..161 / FEATURE_PHASE_158..161 (or set 0) and restart; no persisted state. Documented in PROJECT_ARCHITECTURE.md §10 and docs/TECHNICAL.md §9.1.

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