feat(#91): add Transaction::getMappedRange() for single-round-trip index lookups

[s2x]

Closes #91

Summary

Adds ReadTransaction::getMappedRange() — single-round-trip index lookups backed by fdb_transaction_get_mapped_range / fdb_future_get_mappedkeyvalue_array. Each resolved index row is returned as a MappedKeyValue pairing the index key/value with the records described by the mapper tuple template ({K[N]}, {V[N]}, and the trailing {...} range descriptor).

Notes / findings

• Mapped ranges work only on non-snapshot (read-your-writes) reads in FDB 7.3 (the issue description had this inverted); calling it on Snapshot throws a LogicException.
• The public fdb_c.h FDBMappedKeyValue declaration does not model the real reply layout: the native client returns either a point lookup or a range lookup per row (selected by a variant index at offset 104). The FFI layer decodes both alternatives — details in the issue comment.
• docker/php/Dockerfile now derives the FoundationDB clients deb arch from TARGETARCH (the image previously shipped a mismatched libfdb_c on arm64 builds).

Testing

• composer lint clean (PHPCS + Rector + PHPStan L9)
• composer test:unit — 490 tests pass
• composer test:integration — 239 tests / 813 assertions pass on the 5-node 7.3.75 cluster, including the new MappedRangeTest (7 tests)
• docs/range-reads.md documents the mapper syntax; CHANGELOG.md updated