feat: add 'watchAddress' and 'unwatchAddress' for a real-time address watching system

[ZihaoXue1995]

What this PR adds (Close Issue #50 )

A real-time address watching system for prediction market wallets - the equivalent of CCXT Pro's watchOrderBook but for user's positions, trades, and balances.

New abstractions (core/src/subscriber)

subscriber/base.ts - Core interfaces:

• SubscriptionOption — 'trades' | 'positions' | 'balances'
• SubscribedAddressSnapshot — unified snapshot of trades, positions, balances for an address
• SubscribedActivityBuilder — converts raw event data into a partial snapshot; allows exchange-specific logic to decode on-chain events directly without a REST roundtrip
• BaseSubscriber — interface for pluggable polling/push backends

subscriber/watcher.ts - AddressWatcher orchestrates the CCXT Pro pattern:

• First watch() call returns the initial snapshot immediately; subsequent calls block until something changes (same pattern as watchOrderBook)
• Hybrid fetch: activity builder runs first on event data; only missing types fall back to REST/RPC
• getChanged() computes incremental diffs — only new trade IDs, positions whose size changed, balances whose total changed are dispatched; empty diffs are suppressed
• unwatch() / close() for lifecycle management

subscriber/external/goldsky.ts — Goldsky subgraph subscriber:

• GoldSkySubscriber — polls one or more Goldsky GraphQL endpoints on a configurable interval, with per-address AbortController to cancel stale in-flight requests
• POLYMARKET_DEFAULT_SUBSCRIPTION — orchestrates three parallel + one sequential subgraph query:
• Trades: two indexed queries (maker + taker) merged by ID to avoid the or filter statement timeout
• Positions: PNL subgraph (userPositions) → extract tokenIds → tokenIdConditions for market metadata (avoids the unindexed user + asset_in join that causes timeouts on
  userBalances)
• All queries use orderBy: id to avoid timeouts on unindexed sort columns
• LIMITLESS_DEFAULT_SUBSCRIPTION — two parallel from/to queries for USDC transfers, same or-split pattern
• buildPolymarketActivity, buildPolymarketPositionsActivity, buildLimitlessBalanceActivity — activity builders that decode subgraph payloads into typed Trade[], Position[],
  Balance[]

Exchange changes

Polymarket (watchAddress + unwatchAddress)

• trades — Goldsky CTF Exchange OrderFilled events
• positions — Goldsky PNL subgraph and tokenIdConditions
• balances — on-chain Polygon RPC, USDC.e at 0x2791Bca1f2de4661ED88A30C99A7a9449Aa84174
• fetchPositions(address?) and fetchBalance(address?) now accept an optional address and skip ensureAuth() when offered

Key design

• BaseSubscriber offers basic interfaces to be plugable into AddressWatcher so users can easily replace GoldSkySubscriber with others.
• Indexer is always faster than exchange's restful APIs, so here goldsky is integrated. But goldsky is HTTP-only GraphQL; no Websocket subscription protocol available. (The graph supports websocket subscription with more complicated customized setup)
• Incremental diff dispatch: Callers receive only what changed (new trades, moved positions, shifted balances) and no need to diff on the caller side

Testing (core/examples/social)

• pytest unit tests passed
• Contains track_whale.ts and track_whale.py two testing examples
• Sometimes goldsky subgraph graphql might timeout, users can replace the default graphql endpoint with self-deployed goldsky project endpoint
• On-chain rpc for balance calls failed locally, code remains untouched in the production