Contract, analyzers, and react transforms read nullable backwards

[nathanacurtis]

Subissue of #229.

ADR-065 documents what an absent nullable means on a prop:

• true for StringProp, NumberProp, SlotProp, ImageProp — open value sets
• false for EnumProp — enum enumerates every accepted value

Three consumers predate that and read omission as not-nullable, which inverts the documented default:

• specs/packages/cli/src/transforms/Contract.ts:72 — an omitted nullable emits label: string where string | null is meant, for essentially every non-enum string prop
• specs/packages/cli/src/analyzers/Props.ts:123 — reports omission as non-nullable
• specs-from-figma/packages/react-from-specs/src/React — ignores nullable entirely; only slot props get optional treatment in scaffold.ts:157

Scope

• Apply the per-type default in all three, rather than testing for an explicit true
• Generated contracts will widen to include null for open-valued props — expected, and the correction; regenerate fixtures
• Use nullability when deriving React prop optionality

Depends on

Schema side ships in @directededges/specs-schema 0.29.0 (PR #273).