docs(changeset): say what the $-ban sites actually carry

They keep no annotation: all three read `undecidable` to the detector
rather than `dropped`, so they hold no ledger row and appear in no
`x-dropped-refinements` — published yet unratcheted. One clause; the
arm, the emitter, the ledger and the tests are untouched.

Claude-Session: https://claude.ai/code/session_01AmH9bKvGoLjiY86Q4Z3og2
Co-authored-by: Claude <noreply@anthropic.com>

Author: claude

.changeset/18670-project-banned-keys.md

@@ -21,6 +21,6 @@ One named pattern joins the closed list, and only one:
 
 **⛔ Not a behaviour change, and no document the runtime accepts becomes refused.** The arm is EXACT rather than approximate: a JSON object's properties are exactly its own enumerable string-keyed ones and `propertyNames` judges exactly those names, so "none of the banned names is an own property" and "no property name is one of the banned names" are one sentence read from two ends. It is presence and never value — a banned key present with a `null` value is present on both sides. The accept set at the slot is **unchanged in both directions**: every document the runtime takes (`{}`, `{ "service": "api" }`, any filter carrying no `dialect` key) the file still takes, and every document the runtime refuses the file now refuses too — a `dialect`-bearing object of any shape, the CEL envelope included, since that arm is retired and nothing here revives it. Across the published tree, **1528 of the 1530 per-schema files are byte-identical**; the two that move gain the ban and lose the matching `x-dropped-refinements` row, and nothing else in either file changes.
 
-**The list stays CLOSED.** `packages/spec/src/shared/refinement-projection.ts` declares the vocabulary and builds each predicate from its own declaration — the key list is read once and used by both the published keyword and the enforced rule — so the two cannot name different keys. The predicate judges OWN properties and never `key in value`: `in` walks the prototype chain, so a ban on a name `Object.prototype` carries would refuse `{}` itself while `propertyNames` accepts it, and that is a disagreement about a JSON document rather than an edge outside the domain. A ban over an OPEN set of names — every key starting with `$`, which is what `data/filter.zod.ts`'s normalized field condition refuses — is deliberately not this arm: its keys are a finite list, and a list that merely sampled an open set would be wider than the rule, so those sites stay unprojected and keep their annotation.
+**The list stays CLOSED.** `packages/spec/src/shared/refinement-projection.ts` declares the vocabulary and builds each predicate from its own declaration — the key list is read once and used by both the published keyword and the enforced rule — so the two cannot name different keys. The predicate judges OWN properties and never `key in value`: `in` walks the prototype chain, so a ban on a name `Object.prototype` carries would refuse `{}` itself while `propertyNames` accepts it, and that is a disagreement about a JSON document rather than an edge outside the domain. A ban over an OPEN set of names — every key starting with `$`, which is what `data/filter.zod.ts`'s normalized field condition refuses — is deliberately not this arm: its keys are a finite list, and a list that merely sampled an open set would be wider than the rule, so those sites stay unprojected — and because the detector reads them `undecidable` rather than `dropped`, they carry NO annotation and hold NO ledger row: published yet unratcheted.
 
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