Add POSIX kv tool for parsing key-value pairs

[arthurborgesdev]

Summary: add POSIX kv (jq required) with parse (JSONL) and json (single object).
Behavior: strict fails on first malformed token; loose skips malformed tokens; empty lines emit {}.
Examples:

• echo 'a=1 b=2' | kv parse | jsonl validate
• printf 'Host: example.com\n' | kv parse --delim colon --trim
  Tests: make test
  Limitations: only " and \ unescaped in double quotes; auto header requires standalone key: token.