- rootUnion: an `allOf` member is foldable only when it states nothing beyond
what the merge applies (`type`/`properties`/`required` plus annotations),
so a nested `anyOf`, a `not`, or an `additionalProperties` can no longer
be erased along with the keyword; members are checked against the
ACCUMULATED merge, so two of them contradicting each other is caught even
when neither contradicts the root.
- admitsNull: `const: null` admits null, so a required field pinned to it is
no longer seeded with the one value it accepts and then reported missing.
- collectSchemaDefaults takes the values a caller is about to overlay, and
seeds the branch THEY identify — the App deep link passes its `appArgs`, so
branch 0's defaults can no longer sit invisibly under branch 1's arguments.
- The branch inference moves into core as `selectBranchIndex`, shared by the
seeding and the form's picker so the two cannot disagree.
Signed-off-by: cliffhall <cliff@futurescale.com>
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