[fork-ffi] prelude: represent unit as an interned scalar instead of a table #283

Tracks purescript-lua/purescript-lua-prelude#15 in this tracker.

Problem

The prelude's unit = {} is a reference-typed value: it allocates, carries object identity, and therefore needs the @inline unit never pin (prelude v7.3.1) to stay a shared singleton — the compiler must actively refrain from inlining it, and one dropped pragma away it reallocates per use site. FFI-produced units were never the shared table anyway (foreign modules cannot reference the prelude binding), so identity was already unreliable ecosystem-wide.

Approach

Fix lives in the fork (purescript-lua/purescript-lua-prelude#15): unit = "unit" — an interned scalar is a natural singleton (string interning makes every literal occurrence one object per VM) with zero per-use allocation, purerl's unit-atom move translated to Lua. The pragma flips to @inline unit always, so the binding dissolves and sites carry the literal:

-- today, with the never pin:
local Data_Unit_unit = ({}) -- shared, every use reads the binding
-- after:
-- no binding at all; every site is the literal "unit"

No compiler change is needed: lib/ is representation-agnostic (no reference to Data.Unit anywhere).

Prerequisites / Relations

• Cross-repo prerequisite: Represent unit as an interned scalar instead of a table purescript-lua-prelude#15 must land and release first; this repo then bumps the package set.
• Relates to Optimizer: never duplicate a reference-constructing expression (allocation/identity guard) #284 (allocation/identity guard): either alone suffices for unit — the guard makes the never pin unnecessary for correctness, while this change removes the shared value entirely (the better endpoint).
• The "unit must not be nil" pitfall note in CLAUDE.md / docs/QUIRKS.md should be updated to name the scalar convention for Unit-returning FFI.

Verification / Measurement

On the package-set bump: eval goldens stay byte-identical (they are the check that Array Unit printing survives the representation change); structural goldens churn only where the unit binding/accessor dissolves from linked output. The deterministic TNEW/TDUP census (bench/ci) must not grow at any affected artifact.