Consume prelude v7.3.1: share the unit singleton (@inline unit never) - #258

Picks up prelude v7.3.1 via the package set (psc-0.15.15-20260712-2). The fork flips Data.Unit.unit from @inline unit always to never, which pins the unit = {} foreign singleton to one shared binding instead of letting the accessor fold into every use site. This is the compiler-side half of #176: the fork edit and release landed separately, and this PR just repoints the golden set and accepts the resulting churn.

What moves

• test/ps/spago.yaml + test/ps/spago.lock: the set URL moves to psc-0.15.15-20260712-2, and prelude re-resolves to the v7.3.1 commit.
• Ten golden.lua change shape: where always folded the accessor into each site, never keeps a single local Data_Unit_unit = Data_Unit_foreign.unit and references it by name:

 local Data_Unit_foreign = { unit = {} }
+local Data_Unit_unit = Data_Unit_foreign.unit
 ...
-return { main = Effect_applicativeEffect.pure(Data_Unit_foreign.unit) }
+return { main = Effect_applicativeEffect.pure(Data_Unit_unit) }

• The remaining golden.ir churn is the prelude source path moving to the v7.3.1 commit.

Why it is safe

• Every eval/golden.txt oracle is byte-identical, so runtime output does not change. That includes the Array Unit printing class (Arrays of type Array Unit are always empty #23), which still prints unit / unit / unit / 3.
• Each linked module still holds exactly one {} for unit, inside the hoisted foreign table. The total {} count across the goldens is unchanged.
• Since Foreign table-constructor exports are duplicated per use site (fresh identity and allocation per copy) #175 the compiler already shares foreign tables by default, so this is the declaration of intent that [fork-ffi] prelude: flip @inline unit always to never — inlining unit duplicates the singleton allocation #176 asked for, not a behavioural change. No new test is needed: unit is exercised corpus-wide, and Foreign table-constructor exports are duplicated per use site (fresh identity and allocation per copy) #175's Golden.ForeignSharing eval golden already guards the sharing invariant.

Refs #176