fix: prevent crash on unblessed references in Set::_isdq()

[Koan-Bot]

Summary

Directory::Queue::Set crashed with "Can't call method 'isa' on unblessed reference" when passed an unblessed reference (hashref, arrayref, etc.) instead of producing the expected "not a Directory::Queue object" error message.

Root cause: _isdq() used ref($thing) && $thing->isa(...) — but ref() returns true for unblessed refs too, and ->isa() requires a blessed object.

Fix: Use Scalar::Util::blessed() (core module since Perl 5.8) instead of ref().

Changes

• lib/Directory/Queue/Set.pm: Import Scalar::Util::blessed, use it in _isdq()
• t/2set-validation.t: 5 new tests for various invalid inputs (hashref, arrayref, scalar ref, plain string)

Test plan

• Unblessed hashref now produces "not a Directory::Queue object" error
• Same for arrayref, scalar ref, plain string
• add() method also properly validates
• Full test suite passes (242 tests)

🤖 Generated with Claude Code

[Koan-Bot]

Superseded by #14 — clean branch with only the relevant fix (no fork infrastructure commits).

[Koan-Bot]

Superseded by #14 (clean branch from upstream/master, no fork infrastructure noise).