Fix subsidy-only quickdiff, short hex reads, and logger startup

[f4u57x]

Fixes three issues in CONVOYMining#15, in separate commits:

• Recognize Q...ff as subsidy-only quick-difficulty work, so advertised jobs reach share validation instead of unknown-work.
• Reject truncated hex before reading the next byte.
• Handle logger thread startup failure, clean up resources, and retain console logging.

Adds notify/submit, truncated-input, and logger failure-injection regression tests. Normal and ASan/UBSan builds and full --test suites pass on macOS with ENABLE_API=OFF.