specta: emit Feature.properties as TS unknown instead of recursive Value

[johncarmack1984]

Summary

specta-rc.24's serde_json::Value Type impl is inline: true and recursive (Vec<Value>, Map<String, Value>). When inlined into a struct field like Feature.properties: Option<JsonObject>, the inlined arms reference an undefined Value name in the generated TS:

properties: { [key in string]:
    | "Null"
    | ({ Bool: boolean } & ...)
    | ({ Number: ... } & ...)
    | ({ String: string } & ...)
    | ({ Array: Value[] } & ...)         <- TS2304: Cannot find name 'Value'
    | ({ Object: Map<string, Value> } & ...)
};

Value is never emitted as a top-level alias because its NDT is inline: true, and requires_reference returns false for inline types -- so it's filtered out of the export pass. The inlined references are dangling.

Override the field at the TS layer to specta_typescript::Unknown (renders as TS unknown). Wire format and runtime behavior are unchanged; downstream consumers already have to walk properties opaquely since they're caller-defined per the GeoJSON spec.

Test plan

• cargo build --features specta clean
• cargo test --features specta (37 passed, 0 failed)

[johncarmack1984]

Superseded by the named-JsonValue-alias approach on branch john/specta-json-value-alias (rev 2e34407). The Unknown override discarded structural type information; the new branch emits a proper recursive TS alias that consumers can narrow through.