fix(cpp): handle TS2DIFF float prefixes in batch decode

[kkzi]

Summary

• make C++ FLOAT/DOUBLE TS_2DIFF batch decoding consume the Java-compatible scale/overflow prefix
• route batch reads through the existing segment-aware scalar decoders
• add regression coverage for multiple FLOAT segments and a DOUBLE overflow bitmap prefix

Root cause

The FLOAT/DOUBLE batch overrides delegated directly to the INT32/INT64 TS_2DIFF batch decoders and then bit-cast the results. Those integer decoders expect a delta-block header at the current stream position, but FLOAT/DOUBLE segments place a maxPointNumber or overflow-bitmap prefix before that header. The prefix was therefore decoded as block metadata, leaving the stream and decoder state inconsistent. In tree-reader batch paths this could result in unbounded decoding at end-of-input.

The scalar read_float and read_double implementations already consume the prefix and apply scale/overflow handling. Reusing those implementations restores correctness for normal, overflow, and legacy raw segments. This intentionally gives up the integer SIMD fast path for FLOAT/DOUBLE until a prefix-aware optimized implementation is available.

Tests

• FloatDoubleTS2DIFFCodecTest.*
• TS2DIFFCodecTest.*
• FloatTS2DIFFEncoderResetTest.*
• EncodingCoverage.TS2DIFF*
• TreeQueryByRowTest.QueryByRow_TabletMultiType_PartialPaths
• TsFileWriterTest.WriteDiffrentTypeCombination
• clang-format --dry-run --Werror cpp/src/encoding/ts2diff_decoder.h cpp/test/encoding/ts2diff_codec_test.cc

Fixes #900