Slice C: instance-only qualifying characteristics

[david-w-t]

Slice C — Instance-only qualifying characteristics

A class can now declare a qualifying characteristic (QC) as instance-only:
the attribute is relevant to instances, but binding a value at the class
level is a category error (e.g. a serial_number is meaningful per instance,
never class-wide). The marker is a boolean key instance_only => true
colocated on the class node's QC AVP map:

#{attribute => A, value => undefined, instance_only => true}

Setting the marker

• graphdb_class:add_qualifying_characteristic/3 — (ClassNref, AttrNref, #{instance_only => true}); the existing /2 stays the unflagged declare.
• create_class/3 — accepts an instance-only QC in its initial AVP list.

The marker is currently not settable through update_node_avps/mutate:
slice B's AVP well-formedness check rejects the extra key. That restriction
is a side effect of the slice-B grammar, not a decided long-term contract —
whether toggling a QC's instance-only status should become a first-class
mutation is recorded as an open investigation under the slice-C deferred
follow-ons in TASKS.md.

Enforcement — three class-level value-binding gates

All return the bare reason {error, {instance_only_attribute, AttrNref}}:

Gate	Rejects
bind_qc_value/3	a direct class-level bind on a marked QC
create_class/3	an initial AVP that is instance_only and carries a concrete value
update_node_avps/2	a value-bearing update to a marked stored entry (covers mutate/1 too, both composing update_node_avps_in_txn/3)

Enforcement is local to the class node written. A value-bearing update
includes value => undefined (slice B's upsert would otherwise strip the
marker); a delete is permitted.

Scope / deferred

Inherited enforcement (subclass-redeclare bypass), the per-template attribute
list, template-bound variant values, and the marker-mutability question above
are deliberately deferred and recorded in TASKS.md.

Tests

Full suite green: 488 CT + 133 EUnit = 621, 0 failures, 0 compile
warnings. New: 7 EUnit (class predicates) + 4 EUnit (check_instance_only/2)

• 8 CT across the three gates and the mutate/1 path; reject tests assert the
  full error tuple and prove transaction rollback.

Design: docs/designs/slice-c-instance-only-qc-design.md
Plan: docs/superpowers/plans/2026-06-27-slice-c-instance-only-qc.md

🤖 Generated with Claude Code

[david-w-t]

ok