feat: implement event listeners

[FarajiOranj]

Summary

Closes #121.

Adds event listeners: callbacks that fire as new contract events, blocks, or
signer changes are observed. Each listener polls the underlying adapter on an
interval (as suggested in the issue) and returns a function that stops it.

API

// Contract events (the main ask), e.g. to update a UI on each Transfer.
const unsubscribe = contract.onEvent("Transfer", (events) => {
  for (const event of events) console.log(event.args);
});
unsubscribe();

// Client-level equivalents:
drift.onEvent({ abi, address, event: "Transfer" }, (events) => {/* ... */});
drift.onBlock((blockNumber) => {/* ... */});          // block created
drift.onSignerChange((address) => {/* ... */});       // signer changed

• Listeners are future-only by default; pass fromBlock to also receive
  historical events on the first poll. contract.onEvent applies the contract's
  epochBlock the same way getEvents does.
• pollingInterval and onError are configurable per listener; the interval
  defaults to the adapter's pollingInterval, then 4000ms.
• No changes to Client/createDrift wiring were needed.

Implementation notes

• Listeners poll the adapter directly (not the client's cached getEvents),
  since the cache is keyed and would return stale data to a live listener.
• Polling uses a recursive setTimeout (createPoller) so polls never overlap,
  and a guard prevents any callback from firing after unsubscribe(), even
  mid-poll.

Tests

New createPoller.test.ts and eventListeners.test.ts (block/event/signer
listeners, fromBlock history, in-flight-unsubscribe, throwing-onError
resilience) plus a Contract.onEvent + epochBlock test. Also updated the
README and added a changeset.

Known limitation / follow-ups

• onEvent polls forward by block number and does not handle chain
  reorganizations, so events in blocks re-mined after a reorg may be missed (as
  scoped by the issue's "initial implementation"). This is documented on the
  methods. A reorg-aware mode (confirmations / re-scan window with dedup) would
  make a good follow-up, as would wallet-native signer changed events
  (e.g. accountsChanged) instead of polling.

[changeset-bot]

🦋 Changeset detected

Latest commit: fbd5956

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 5 packages

Name	Type
@gud/drift	Minor
@gud/drift-ethers-v5	Minor
@gud/drift-ethers	Minor
@gud/drift-viem	Minor
@gud/drift-web3	Minor

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