fix(downgrader): inline $refs into keywords replaced by 3.1 to 3.0

[dinwwwh]

Problem

danglesIn keeps a $ref while its location still exists in the output. It treated only a missing location, or the items: {} placeholder, as gone. But 3.1 → 3.0 writes new values over some converted keywords, so a $ref into one of them was kept and resolved to the new value:

{
  properties: { a: { type: 'null', enum: ['x'], not: { type: 'string' } } },
  not: { $ref: '#/properties/a/not' },
}

a matches nothing, so the converter gives it not: {} in place of not: { type: 'string' }.

	root not	accepts 1
3.1 input	{ type: 'string' }, through the $ref	yes
main	{ $ref: '#/properties/a/not' }, which now finds {}	no
this PR	{ type: 'string' }, inlined	yes

The same happens in components.schemas, and for two more keywords:

• const replaces the enum beside it. The new enum has the same length, so the index check missed it, and a $ref to enum/0 got the const value.
• "null" in type sets nullable: true over a 3.0 nullable: false.

Fix (packages/downgrader/src/shared.ts)

• replace(target, key, value) writes a value that stands in for the source's instead of converting it, and tags key on target, with one WeakSet per key name.
• danglesIn treats a tagged key like a removed one, so the $ref is inlined with the converted original.
• not: {}, enum from const, nullable: true, and the items: {} placeholder now go through replace(). It replaces placeholder(), which tagged the value and so could not mark an entry inside it, such as enum/0, or a primitive, such as true.

Tests

• schema/references.test.ts: a $ref to each replaced keyword, and to an enum replaced by a null const and then narrowed by a null-only type.
• spec/components.test.ts: the not case in components.schemas.
• All five fail on main. pnpm test (697 passed), pnpm lint and pnpm type:check are clean.

Known limits

• Overwrites that only invalid input reaches still keep the $ref: required: true over a path parameter's required: false, minimum or maximum over a value that is not a number, and XML attribute or wrapped over false.

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[pullfrog]

your Pullfrog Router balance is empty, and this repo has no provider key to fall back on, so the agent never ran.

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[dinwwwh]

The pullfrog check failing on 77c355a is not caused by this PR. Its log stops at action failed: your Pullfrog Router balance is empty, and this repo has no provider key to fall back on, so the agent never ran, before any review starts. The same check fails the same way on every PR since at least #40, including runs against main's head.

No change in this PR can fix it. It needs a Pullfrog top-up or a provider key in the repo's secrets. I'm not re-running it, because it would fail on the same billing error. The repo's own CI (lint_and_typecheck and test_matrix) is what gates this PR.

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Top up Router → · Model settings → · Setup docs → · Ask in Discord →

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