docs: document scanIterator()/scanPrefixIterator() (#391)

[s2x]

Description

Documents RawKvClient::scanIterator() / scanPrefixIterator() (issue #391, audit finding DOC-25) — previously the only mention anywhere was a line in the README file tree, while the docs taught a hand-rolled ScanPaginator whose incrementKey() produced a smaller end key for 0xFF-terminated prefixes (silently empty scans).

All claims verified against src/Client/RawKv/ScanIterator.php, RawKvScanner.php and RawKvClient.php.

Closes #391

Changes

• docs/operations.md: new Iterating Large Ranges subsection — both signatures (batchSize = 256 default, bounds 1 <= batchSize <= 10240 validated synchronously with InvalidArgumentException), keyOnly null-value behaviour, page/continuation mechanics (one page in memory, continues from lastKey . "\x00", exhausts on a short page), rewindable re-scan-from-start, lazy RPCs/exceptions during iteration, no reverse iterator; the manual pagination loop in the Scan section is replaced by the iterator.
• docs/advanced.md: the hand-rolled ScanPaginator class is deleted and replaced with the built-in iterators, plus an explicit warning about the wrong chr($lastByte + 1) prefix-end arithmetic vs RawKvSplitter::calculatePrefixEndKey().
• README.md: iterator examples in the Scanning section and a bullet under Implemented Operations → Scanning.
• docs/troubleshooting.md: the 10240 scan-limit row now cross-references the iterators.
• CHANGELOG.md: new ### Documentation entry under [Unreleased] ([DOC-25], [DOC-25] Document scanIterator() / scanPrefixIterator() #391).

Changelog

• [DOC-25]: scanIterator() / scanPrefixIterator() documented in docs/operations.md (Iterating Large Ranges) with signatures, batchSize bounds (1..10240), keyOnly null values, page/continuation/rewind semantics and lazy-exception behaviour; the buggy hand-rolled ScanPaginator in docs/advanced.md and the manual pagination loop in docs/operations.md are replaced with the built-in iterators, cross-referenced from README and the troubleshooting scan-limit row. ([DOC-25] Document scanIterator() / scanPrefixIterator() #391)

Code Review

• Pending subagent code review