fix: preserve numeric results, file data, and I/O deadlines

[LunaStev]

This fixes incorrect floating-point results, destructive same-file copies, incomplete bounded file reads, child pipe leaks, thread-only process exits, and socket operations that could outlive their timeout. Each issue has its own signed-off commit and regression coverage.

• Preserve exact integers and signed zero when rounding, reject invalid approximate comparisons, avoid interpolation overflow between finite endpoints, and retain subnormal results for negative integer powers.
• Read bounded files through EOF without depending on seekable size metadata. Compare opened file identities before truncating copy destinations, including hard links and symbolic links.
• Close unused capture readers before child descriptor remapping and use Linux exit_group to terminate every thread.
• Enforce timed socket I/O with per-call nonblocking operations on Unix and cancellable overlapped operations on Windows, without changing shared socket modes.
• Add native I/O regression fixtures and include their harness in the Windows ARM64 CI runtime step.

Validation:

• Workspace/all-target Rust tests: 197 passed; the native harness also passed 20 I/O scenarios on Linux.
• Clippy with warnings denied, formatting, standard-library policy checks, and whitespace checks passed.
• All 609 case source checks and four affected existing Linux cases passed.
• Math regressions passed at O0/O2/O3, compiled for all 21 supported target triples, and ran under AArch64, RISC-V64, and LoongArch64 QEMU.
• I/O object generation passed across 12 target triples. Supplementary Windows GNU/Wine checks covered file identity, bounded reads, timed I/O, cancellation, and IOCP completion suppression.
• Each intermediate commit passed its corresponding regression checks.

Native macOS, FreeBSD, and Windows execution remains for platform hosts/CI; Wine checks do not replace native validation. Windows timed I/O requires overlapped-capable sockets. On cancellation errors Winsock does not report partial transfer counts, so returned counts describe confirmed completions.

Fixes #554
Fixes #555
Fixes #556
Fixes #557
Fixes #553
Fixes #552
Fixes #559
Fixes #565
Fixes #510