Meta-Nu transformations

Rewrites that map a Nu to an equivalent Nu.

Equivalence

Two Nus are equivalent iff they have the same mutation on the Context for any starting Context. Rewrites must preserve equivalence.

Rewrite	Example
Constant folding	`Add(2, 3)` → `Literal(5)`
Common-subexpression	shared `ref.fetch()` collapsed to one read
Independence parallelization	`a >> b` where independent → `a \| b`
Identity elimination	`Add(x, 0)` → `x`

Transformations operate on Nu trees, producing Nu trees. They don't run Nus; they reshape them. Composition rules bound which reshapes are valid.