Harden pool protocol handling and add CI

[JustAResearcher]

Summary

• cap pool response lines at 1 MiB and parse them incrementally without rescanning buffered data
• sanitize and truncate pool-controlled terminal output
• serialize socket sends and teardown, with shutdown -> join -> close ordering
• reject malformed or unsupported pool targets instead of silently falling back to difficulty 1
• add cross-platform regression tests and baseline GitHub Actions CI

Field-test baseline

The published v1.1.3 artifacts were tested before these changes:

• Windows RTX 5090: official ZIP, 480 seconds at 50% intensity, 11.043 g/s, 2 accepted, 0 rejected, 0 verify failures, clean exit
• Linux RTX 4070 Ti SUPER: official tarball, about 5.62 g/s at 50% intensity, 1 accepted, 0 rejected, clean TERM shutdown with no orphan process

Validation

• Windows MSVC protocol regression suite: passed
• Linux g++ protocol regression suite: passed
• existing Tari wrapper self-test: passed on Windows and Linux
• Windows PowerShell and Linux/HiveOS shell syntax checks: passed
• Windows CUDA 13.2 builds: sm_86, sm_89, sm_120 passed
• Linux CUDA 12.8 builds: sm_86, sm_89, sm_120 passed; maximum required GLIBC is 2.17
• exact sm_120 binary loopback tests: oversized line, malformed target, ESC/C1 terminal controls, clean shutdown; all passed
• hardened RTX 5090 live run: 300 seconds at 50% intensity, 10.852 g/s, 1 accepted, 0 rejected, 0 verify failures, no orphan process
• local Windows/Linux/HiveOS package assembly and executable-mode checks: passed (not published)

Closes #6
Closes #7
Closes #8
Closes #15