fix: resolve all compiler warnings in Release build

[osamu620]

Summary

• Remove unused variables: x_acc in coding_units.cpp, stride4 and t2 in ht_block_encoding_avx512.cpp
• Check fread return value instead of discarding it (main_jpip_benchmark.cpp)
• Add explicit static_cast<size_t> for int-to-size_t sign conversion (j2kmarkers.cpp)
• Fix misleading indentation: split same-line if/assignment onto separate lines (jpip_index_check/main.cpp)
• Add static_cast<ptrdiff_t> for size_t-to-difference_type conversion (jpip_parser_check/main.cpp)

Clean Release build now produces zero warnings with GCC.

Test plan

• cmake --build produces zero warnings
• 402/402 tests pass

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Cleans up GCC Release-build warnings by removing unused variables, checking a previously discarded fread return, adding explicit casts for sign/width conversions, and reformatting a confusingly-indented if/assignment chain.

Changes:

• Remove unused locals (x_acc, stride4, t2).
• Add explicit static_cast for sign and ptrdiff_t conversions; check fread return value.
• Split combined if/assignment statements onto separate lines in the JPIP index check parser.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
source/core/coding/coding_units.cpp	Drop unused x_acc in line-based encoder loop.
source/core/coding/ht_block_encoding_avx512.cpp	Drop unused stride4 constant and t2 temporary in AVX-512 cleanup encode.
source/core/codestream/j2kmarkers.cpp	Wrap TLM entry_size expression in static_cast<size_t> to silence sign-conversion warning.
source/apps/jpip_benchmark/main_jpip_benchmark.cpp	Check fread return value and return empty buffer on partial read.
tests/tools/jpip_index_check/main.cpp	Split same-line if/assignment statements into separate lines.
tests/tools/jpip_parser_check/main.cpp	Cast iterator offset to ptrdiff_t for sign-conversion compliance.

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