conflicts="--b --c" is accepted but silently unenforced #1598

Issue drafted by Cluade Sonnet 5.5

Version: usage 7.1.3

Repro:

#!/usr/bin/env -S usage bash
#USAGE flag "--a" conflicts="--b --c"
#USAGE flag "--b"
#USAGE flag "--c"
echo ok

./script --a --b and ./script --a --c both print ok.
Expected: error Invalid flag --a: conflicts with --b.

Observed:

• conflicts="--b" works.
• conflicts="--b --c" loads without error and enforces nothing.
  The whole string appears to be treated as a single flag name.
• Repeating the property (conflicts="--b" conflicts="--c") enforces only the last one.
• The child-node form flag "--a" { conflicts "--b" "--c" } works as expected.

Expected:

• Either parse a space-separated list in the property, or reject it at spec-load time (for example, "conflicts references unknown flag --b --c").
• The flag reference page documents only the single-value property form.
  Please also document the child-node form for several conflicts.