feat(#92): add Future::awaitAll/onReady and range read-ahead

[s2x]

Closes #92

Implements the first, self-contained slice of #92 (per the issue's own note that read-ahead + group await deliver most of the practical win without the callback binding):

Added

• Future::awaitAll(array $futures): array — resolves a batch of futures that were all issued up front. Since every future is already in flight, awaiting them in order costs roughly one round trip of total latency, not N.
• Future::onReady(callable $fn): void — PHP-thread completion hook (runs immediately for a ready future, otherwise blocks then invokes).
• Deliberately NOT bound: fdb_future_set_callback — its callback fires on the FDB network thread, where executing PHP is unsafe (same reasoning as the existing NativeClient::onNetworkThreadCompletion). Documented in docs/advanced.md.

Changed

• RangeResult::paginate() now issues the request for the next chunk before yielding any item of the current chunk (read-ahead, mirroring Java's AsyncIterable). Read-ahead never fetches past an explicit limit; iteration order, dedup guarantees and pagination semantics are unchanged.
• New KvResultFuture interface (implemented by FutureKeyValueArray) describes the pagination contract and keeps paginate() unit-testable without a cluster.

Tests

• tests/Unit/FutureAwaitAllTest.php — stub-lib unit tests for awaitAll() / onReady() (ready & not-ready paths, empty batch, non-Future rejection).
• tests/Unit/RangeResultTest.php — new tests proving the next-chunk request is issued before the first item of the current chunk is consumed, and that no page beyond the limit is requested.
• tests/Integration/FutureAwaitAllIntegrationTest.php — live-cluster coverage: 50 parallel reads resolved via awaitAll(), mixed future types resolved in order.

Docs & changelog

• docs/advanced.md: new "Group Awaits and Completion Hooks" section (async model and its limits).
• CHANGELOG.md: entries under Unreleased → Added, referencing Add non-blocking future support (fdb_future_set_callback, awaitAll, range read-ahead) #92.

CI: composer lint (PHPCS + Rector dry-run + PHPStan level 9) and composer test (unit + integration against a 5-node Docker FDB cluster) are clean locally.

Remaining from #92 (not in this PR): Fiber-aware await and any eventual safe callback binding — these stay as follow-up work on the issue.

[s2x]

Closing as superseded by #111 — the same functionality (Future::awaitAll()/onReady(), RangeResult read-ahead, KvResultFuture, unit/integration tests, docs and CHANGELOG entries) was already squash-merged to master in commit df4e86e. No additional commits on this branch; nothing here is lost. Remaining follow-ups from #92 stay on the issue.