feat: add opt-in query logging

[Artmann]

Summary

Adds opt-in logging of every MongoDB operation esix runs — operation name, collection, arguments, duration, and error (if any). Two ways to enable it:

// Programmatic — plug in any logger
import { setQueryLogger } from 'esix'

setQueryLogger((entry) => {
  myLogger.debug(entry)
})

# Or via env var — built-in console.debug output:
# esix invoices.insertOne([{...}]) took 1.2ms
DB_LOG_QUERIES=true

How it works

• All DB access already funnels through useCollection() in the QueryBuilder, so the collection is wrapped there in a logging Proxy — one choke point, no changes to any query method.
• Promise-returning ops log when they settle; cursor ops (find, aggregate) log when toArray() settles, so duration covers the full operation and survives sort().skip().limit() chaining.
• Errors are logged with the error field set and the original rejection propagates unchanged.
• A throwing user logger can never affect a query: logger calls are guarded, so a faulty logger neither rejects a successful operation nor masks a real DB error (regression-tested).
• Zero overhead when disabled: no Proxy is allocated and the raw collection is used.

Changes

• packages/esix/src/query-logger.ts (new) — QueryLogEntry/QueryLogger types, setQueryLogger(), logger resolution, and the collection/cursor proxies.
• packages/esix/src/query-builder.ts — wraps the collection in useCollection() only when a logger is active.
• packages/esix/src/index.ts — exports setQueryLogger, QueryLogEntry, QueryLogger.
• packages/esix/src/query-logger.spec.ts (new) — 13 tests: integration against the mock adapter (entries for create/find/save/aggregate, env-var console path, disabled-mode silence, custom-logger precedence) and unit tests for the proxy (error propagation, chained-cursor single log, property passthrough, throwing-logger guards).
• packages/website/docs/configuration.md — DB_LOG_QUERIES env entry and a Query Logging section.

Testing

• yarn lint, yarn typecheck, yarn test all green (248 tests, +13 new).
• Reviewed by a fresh-context review pass before opening this PR; its one finding (an unguarded user logger could break queries) is fixed and regression-tested.

Closes #28

🤖 Generated with Claude Code

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