fix(index-block): validate shared prefix against restart-key span

[polaz]

Context\nFound during review of PR #182 (CodeRabbit duplicate body item).\n\n## Problem\n currently validates against the current entry boundary, but it does not verify that the shared prefix stays within the actual restart-key span. With malformed on-disk data this can still produce semantically invalid key reconstruction inputs.\n\n## Proposed approach\nCarry restart-key span (start+end/len) through decoder state and validate before constructing .\n\n## Scope\n- src/table/block/decoder.rs\n- src/table/index_block/block_handle.rs\n- src/table/index_block/mod.rs\n- tests for malformed truncated entries\n