leveldb: correct trailer boundary check in Reader::SkipToInitialBlock #33955

replaces the magic number 6 in the trailer boundary condition with kHeaderSize and corrects an off-by-one by expressing the rule as offset_in_block > kBlockSize - kHeaderSize, which is equivalent to offset_in_block >= kBlockSize - 6. According to src/leveldb/doc/log_format.md: “A record never starts within the last six bytes of a block” and when exactly seven bytes remain a record may start (e.g., a zero-length FIRST), so the equal case must be treated as trailer. The writer already uses kHeaderSize to manage trailers, and aligning the reader with kHeaderSize removes a format-coupled magic number and ensures future-proof consistency. Functionally this avoids scanning a block when initial_offset_ is in the last six bytes, matching the documented format; it was previously a minor inefficiency and comment mismatch rather than a correctness issue.

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Code Coverage & Benchmarks

For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/33955.

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thx, but this will need to go upstream