Add core observability events

[AllyMurray]

Summary

Adds structured lifecycle observability to @http-client-toolkit/core via HttpClientOptions.observability.onEvent.

What Changed

• Added the exported HttpClientEvent discriminated union and event types for requests, cache, dedupe, rate limiting, server cooldowns, and retries.
• Instrumented HttpClient lifecycle events while swallowing observer errors and detaching async observer promises so callbacks cannot change request results.
• Added focused tests for event ordering, cache hit/miss/stale/revalidation paths, retry scheduling/exhaustion, rate-limit wait/throw behavior, dedupe owner/join behavior, and observer error/non-await semantics.
• Updated the core README and docs site with usage examples, stable payload guarantees, and logging/metrics/OpenTelemetry bridging guidance.
• Added a minor changeset for @http-client-toolkit/core.

Validation

• pnpm --filter @http-client-toolkit/core test passed: 314 tests
• pnpm --filter @http-client-toolkit/core exec tsc --noEmit passed
• pnpm --filter @http-client-toolkit/core build passed
• cd docs-site && pnpm --ignore-workspace build passed
• git diff --check passed
• Pre-push hook passed monorepo build, lint, and test tasks