Axis behavior native verification
Base: 7db05401d0d25f37143d875488c7a5a1b1720248
Head: 84743ae02302b7ad02513a429262e30d1024dd6b
Xcode 27.1 build 27A9269; isolated FabricExample, Metro 18133.
UIKit raw values: automatic=0, horizontalOnly=1, verticalPreferred=2.

Actual base native items:
[{"items":[{"axis":0,"custom":false,"identifier":"axis-native"},{"axis":0,"custom":true,"identifier":"axis-custom"}],"title":"Axis one"}]
Actual head native items:
[{"items":[{"axis":1,"custom":false,"identifier":"axis-native"},{"axis":2,"custom":true,"identifier":"axis-custom"}],"title":"Axis one"}]

iOS 27.1 live navigation-item assertions:
PASS initial explicit preferences {"axis-native": 1, "axis-custom": 2}
PASS Native automatic {"axis-native": 0, "axis-custom": 2}
PASS Native verticalPreferred {"axis-native": 2, "axis-custom": 2}
PASS Native horizontalOnly {"axis-native": 1, "axis-custom": 2}
PASS Custom horizontalOnly {"axis-native": 1, "axis-custom": 1}
PASS Custom automatic {"axis-native": 1, "axis-custom": 0}
PASS Custom verticalPreferred {"axis-native": 1, "axis-custom": 2}
PASS Remove axis options {"axis-native": 0, "axis-custom": 0}
PASS Native horizontalOnly {"axis-native": 1, "axis-custom": 0}
PASS Custom verticalPreferred {"axis-native": 1, "axis-custom": 2}
PASS remove both items {}
PASS reinsert explicit preferences {"axis-native": 1, "axis-custom": 2}
PASS fixed spacer retains preferences {"axis-native": 1, "axis-custom": 2}
PASS remove fixed spacer {"axis-native": 1, "axis-custom": 2}
PASS native and custom callbacks
PASS same identifiers on pushed screen {"axis-native": 1, "axis-custom": 2}
PASS update pushed screen {"axis-native": 2, "axis-custom": 2}
PASS pop restores original screen preferences {"axis-native": 1, "axis-custom": 2}

iOS 18.6 live UI assertions:
PASS iOS 18.6 Native verticalPreferred retains default horizontal placement
PASS iOS 18.6 Custom horizontalOnly retains default horizontal placement
PASS iOS 18.6 Native automatic retains default horizontal placement
PASS iOS 18.6 Custom automatic retains default horizontal placement
PASS iOS 18.6 Remove axis options retains default horizontal placement
PASS iOS 18.6 reinsert omitted preferences
PASS iOS 18.6 native and custom callbacks

Full FabricExample base and head builds: PASS.
Content-factory translation-unit syntax checks: PASS against iOS 27.1, iOS 27.0, tvOS 27.1, visionOS 27.1.
Older SDK was installed Xcode 27.0 build 27A5209h.
Full Android/tvOS/visionOS app builds were not run.