[akasakariko]

Fixes #1361

Summary

• PosixRandomAccessFile::Read previously issued a single pread syscall, treating legal positive short reads as complete reads and failing on reads exceeding Linux's 0x7ffff000 per-call limit
• PosixRandomAccessFile::Read now loops pread calls until the requested buffer is filled, EOF is reached, or an unrecoverable non-EINTR error occurs
• Advances buffer pointer and offset by accumulated bytes read across iterations
• Retries automatically on EINTR without advancing buffer offset
• Returns already-read bytes in the result slice even if a subsequent read call errors
• Preserves temporary file descriptor closing on all exit paths
• Adds Linux-only link-time pread wrapping via -Wl,--wrap=pread in static env_posix_test without public API changes or production hooks
• Adds 18 parameterized regression tests covering permanent and temporary descriptors, short reads, interrupted reads, EOF, zero-length requests, and partial errors

Testing

• All tests executed locally; no remote GitHub CI run has been triggered yet
• Regression test reproduces the failure prior to the fix: 10 of 18 test cases failed with short reads returning truncated data ("cde" instead of "cdefghijkl")
• Regression test passes with fix applied: 18/18 test cases pass
• Sanitizer stress test passes: 100 shuffled repetitions pass under Clang with ASan, UBSan, and leak detection enabled
• Full CTest test suites pass across local builds rebased on bb74ef7:
  • Linux GCC Debug static: 3/3 targets pass
  • Linux GCC Release: 3/3 targets pass
  • Linux Clang Debug static with ASan, UBSan, and leak detection: 3/3 targets pass
  • macOS Clang Debug static: 3/3 targets pass
  • macOS Clang Release shared: 2/2 targets pass (env_posix_test excluded upstream in shared builds)
• Release build and install succeed: benchmark binaries compile and cmake --install completes cleanly
• Code style and hygiene checks pass: git diff --check and clang-format on changed lines pass with clean diff
• Compiler warning checks pass: clang++ with -Wall -Wextra -Werror -Wno-unused-parameter -Wthread-safety passes on modified files util/env_posix.cc and util/env_posix_test.cc

Limitations

• Did not run the original >2 GiB / ~12 GB RAM end-to-end workload because the local test VM has ~2 GiB RAM; deterministic syscall wrapping was used instead to reproduce short reads safely
• Local testing only; GitHub CI integration has not run
• No testing performed on Windows or 32-bit platforms
• Benchmark binaries were compiled only and not executed
• Optional compression test cases skipped where libraries were absent: one Zstd test on Linux, and both Snappy and Zstd tests on macOS static build
• Project-wide -Wall -Wextra with -Werror fails on pre-existing warnings in unmodified source files; standard project build flags pass cleanly