fix(core): register Rain, Gemini Titan, and Hyperliquid defaults

[realfishsam]

Summary

• Adds rain, gemini-titan, and hyperliquid to the sidecar default exchange registry.
• Refactors the registry into an exported DEFAULT_EXCHANGE_NAMES list so coverage can be regression-tested.

Fixes #1158

Test Plan

• npm test --workspace=pmxt-core -- --runTestsByPath test/pipeline/default-exchanges.test.ts --runInBand (verified RED before fix, PASS after fix)
• npm test --workspace=pmxt-core -- --runTestsByPath test/pipeline/error-propagation.test.ts --runInBand
• npm run build --workspace=pmxt-core

Note: the first local build attempt failed because dependencies were stale/missing (@buidlrrr/rain-sdk absent from local node_modules); after npm install, the build passed with no lockfile changes.

[realfishsam]

Local verification passed for the scoped fix:

• npm test --workspace=pmxt-core -- --runTestsByPath test/pipeline/default-exchanges.test.ts --runInBand
• npm test --workspace=pmxt-core -- --runTestsByPath test/pipeline/error-propagation.test.ts --runInBand
• npm run build --workspace=pmxt-core after refreshing dependencies with npm install

Current required generated-sync checks are red, but the failed diffs are broad pre-existing/generated drift outside this focused issue (API reference adds sourceExchange/exchange, TS reference removes question, docs/llms samples swap Router/Kalshi and version metadata). I’m keeping this PR scoped to #1158 rather than committing unrelated generated churn into the default-exchange registry fix.

[realfishsam]

PR Review: PASS (NOT VERIFIED)

What This Does

Adds Rain, Gemini Titan, and Hyperliquid to an exported DEFAULT_EXCHANGE_NAMES sidecar registry constant and builds the singleton registry from that list. This gives the default exchange coverage a direct regression test and makes the intended unauthenticated public-read venues explicit.

Blast Radius

Core sidecar server registry only: core/src/server/app.ts, plus a new pipeline test. No SDK wrappers, OpenAPI schemas, exchange normalizers, or router merge logic are changed.

Consumer Verification

Before (base branch):
core/src/server/app.ts hard-coded defaultExchanges without Rain/Gemini Titan/Hyperliquid entries:

const defaultExchanges: Record<string, any> = {
  polymarket: null,
  limitless: null,
  kalshi: null,
  ...
  mock: null,
};

After (PR branch):
The server exports and tests a canonical list containing the three public-read venues:

export const DEFAULT_EXCHANGE_NAMES = [
  "polymarket",
  ...
  "rain",
  "gemini-titan",
  "hyperliquid",
  "mock",
] as const;

I did not mark this VERIFIED because a live fetchMarkets consumer smoke against these public venues was not conclusive in this environment: network/API responses timed out rather than producing stable venue data. Static coverage and the targeted registry test do confirm the intended registry change.

Test Results

• Build: PASS (npm run build --workspace=pmxt-core)
• Unit tests: PASS (npm run test --workspace=pmxt-core -- --runTestsByPath test/pipeline/default-exchanges.test.ts, 1 passed)
• Server starts: PASS (health endpoint responded during local smoke setup)
• E2E smoke: NOT VERIFIED (public venue fetchMarkets was not stable/conclusive in this environment)

Findings

No blocking findings.

PMXT Pipeline Check

• Field propagation (3-layer): N/A
• OpenAPI sync: N/A
• Financial precision: N/A
• Type safety: OK
• Auth safety: N/A

Semver Impact

patch -- sidecar registry/test coverage fix with no public API shape change.

Risk

The PR verifies membership in the default registry but does not itself prove Rain/Gemini Titan/Hyperliquid public reads succeed against live upstream APIs; those remain dependent on each exchange implementation/upstream availability.