skills: no rule holds an absence assertion to the encoding it searches #1306

Problem

An assertion that a spelling is absent from encoded output cannot fail when the spelling differs from what the encoder writes. morphic-verifying-changes says what a test must reach (references/assertions-and-reach.md, references/tests-that-bite.md), and no rule names this shape:

$ git grep -n NotContains -- .claude/skills/morphic-verifying-changes | wc -l
0

A test written for #1302 asserted that no Constraints held a bound:

assert.NotContains(t, marshalForTest(t, doc), `"minItems": 1`, "no Constraints anywhere holds the bound")

marshalForTest encodes compact JSON, so "minItems":1 is what a document holds and the spaced spelling matched nothing, on any document. It was found by a planted defect that wrote the bound onto a Scalar the census also keeps verbatim:

SURVIVED census alias also reads collection bounds | 0

With the assertion reading the node's field instead, the same plant is killed:

KILLED   census alias also reads collection bounds | TestCollectionBound_ANodeOfAnotherShapeKeepsItVerbatim

The rule's absence is what let the assertion read as a check. It was the only instance: once it was fixed, this finds none on the branch's tree.

git grep -nE 'NotContains\(t, .*`"[A-Za-z:_]+": ' -- '*_test.go'

Direction

A proposal: a rule in references/assertions-and-reach.md. Assert absence on the field that would hold the value, not on an encoding of the document. Where the encoding is the subject, assert the same spelling's presence in a control, so the search is shown to match. The probe is to plant the value and watch the assertion redden.

Out of scope

Presence assertions, which fail on a mismatched spelling and so announce themselves.

Related: #1302

Priority: P4, rule 1 (housekeeping: a skill rule, no change to lowering, IR or emit contract). git grep -n NotContains -- .claude/skills/morphic-verifying-changes | wc -l → 0 (morphic @608a5a46).