Add contrastive-comma pattern for the "X, not Y" aphorism

[aaazzam]

What

Sibling to the existing contrastive pattern. Catches the standalone "X, not Y" construction — same rhetorical move as "it's not X, it's Y" but structurally distinct, so the existing pronoun-led regex misses it entirely.

Why

This is the most common form of contrastive parallelism in modern AI prose. A recent in-house blog post draft had five instances and dslop fired on zero of them. The construction is highly diagnostic when it terminates a clause — that's where it's working as a slop-shaped aphorism rather than as a legitimate qualifier.

Examples

Flagged (clause-terminating aphorisms):

Design your tools as verbs, not nouns.
They are complements, not substitutes.
We built a schema, not a product.
It's a feature, not a bug.
Ship verbs, not nouns -- the model handles the rest.

Not flagged (clause continues after Y, so it's a real qualifier):

I bought milk, not cream, from the store.
He said, not without reason, that we should leave.
The tests pass, not because of luck but because of design.
We need water, not soda or juice or tea.
If you want bread, not toast, ask the baker.

Guardrail

The regex requires the construction to terminate the clause:

• ., !, ?, ;
• em-dash (U+2014) or ASCII --
• end-of-line

That envelope catches aphoristic uses and excludes the legitimate "I want X, not Y, but rather Z" mid-sentence pattern.

Validation

Smoke-tested against three real Prefect prose docs:

File	contrastive-comma hits	Verdict
architecture-of-agency blog post	3	All true positives ("jobs, not schemas," "complements, not substitutes," "verbs, not nouns")
mcp-oauth resource (~2k words)	0	Clean — long-form technical prose doesn't trip the rule
best-mcp-deployment-platforms resource (~6k words)	0	Clean

Default: true (matches contrastive). Can be disabled per-repo via dslop.toml if FPs surface in different domains.

Files

• src/patterns.rs — new CONTRASTIVE_COMMA pattern + detect_contrastive_comma function, registered in active_patterns
• src/config.rs — new contrastive_comma: bool in PatternConfig + RawPatterns, default true
• evals/conftest.py — new contrastive-comma EvalCase, exercises the harness's "can a model fix it given the error" check
• README.md — updated description and config example

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