Cleanup Release

Title: v1.0 — Full Code Audit & Bug Fix Release

Body:

Summary

Complete security and correctness audit of the entire codebase. 66 issues identified and resolved across src/, console2udp/, build scripts, and documentation.

| Severity | Count | Status    |
 |----------|-------|-----------|
 | Critical | 7     | All fixed |
 | High     | 14    | All fixed |
 | Medium   | 20    | All fixed |
 | Low      | 8     | All fixed |

Highlights

• Checksum calculation was broken — only used last byte instead of additive sum
• Buffer underflow in padd_set_string() wrote before buffer start
• Signal handler called non-async-safe functions like printf and syslog
• Format string crash — printf("%s", int_value) segfault in MTU test
• Inverted strcmp logic caused HTTP response parsing to reject valid responses
• Stack overflow in curl_get via unbounded sprintf/strcat chain
• Shell script infinite recursion — kill() called itself instead of system kill
• Compiler hardening flags added
• Hold time arithmetic guarded against U32 wraparound

See https://github.com/lowerpower/SainSmartUsbRelay/blob/master/CHANGELOG.md for the complete fix list.