clippy: box the `Value::Closure` payload (large_enum_variant, currently allowed)

[hyperpolymath]

Surfaced by PR #358, which revived Rust CI after it had failed 60/60 runs at 0s without
ever being parsed by GitHub. With the gate running again, clippy -D warnings reports:

error: large size difference between variants
   --> src/ephapax-interp/src/lib.rs:110:1
    | Closure { param, param_ty, body, env }  -- largest variant, >= 264 bytes
    | String  { data, region }                -- second largest, >= 48 bytes

Value::Closure is roughly 5.5x the next largest variant, so every Value — every
Unit, Bool, I32 — costs 264 bytes in the interpreter's hot path.

Why it is currently #[allow]ed

The fix is to box the closure payload. That changes every construction site and every
match arm for Value::Closure across the interpreter — the semantics-bearing component
of the language. Bundling that refactor into a CI-infrastructure PR would have hidden a
real behavioural-risk change inside an unrelated diff, so PR #358 instead added a narrow,
commented #[allow(clippy::large_enum_variant)] at the enum.

This issue tracks doing it properly.

Suggested approach

Box the payload, e.g.:

Closure(Box<ClosureData>),

pub struct ClosureData {
    pub param: Var,
    pub param_ty: Ty,
    pub body: Expr,
    pub env: Environment,
}

Then remove the #[allow] at src/ephapax-interp/src/lib.rs.

Acceptance

• cargo clippy --all-targets -- -D warnings passes with the #[allow] removed.
• Interpreter tests pass unchanged — this must be a pure representation change with no
  observable semantic difference.