Compute vector magnitude with `magnitude`

[Seddryck]

Summary

Add a magnitude operation for the first-class numeric Vector type introduced by #684.

magnitude returns the Euclidean (L2) magnitude of the input vector.

Syntax

V(3, 4) | magnitude

returns:

5

Semantics

For V(x1, ..., xn), the result is:

sqrt(x1² + ... + xn²)

• Input: Vector
• Output: numeric
• An all-zero vector has magnitude 0.
• Numeric behavior follows existing Expressif promotion/coercion rules.
• Generic arrays and tuples are not implicitly treated as vectors.

Naming

Use magnitude rather than the former norm wording from #553. This keeps the user-facing vocabulary explicit and avoids overloading norm with different possible norm definitions.

Acceptance criteria

• V(3,4) | magnitude returns 5.
• Zero, positive, and negative components are supported.
• The all-zero vector returns 0.
• Result precision follows Expressif numeric semantics.
• Generic arrays and tuples are rejected as vector input.
• Tests and function metadata/documentation are added.

Dependency

Depends on #684.