Add named math constants (PI, E, TAU)

[timfennis]

Summary

The language currently has no named constants for common mathematical values. Users have to write magic number literals like 3.14159265358979... when working with trigonometry.

Proposed constants

• PI (or pi) — π ≈ 3.141592653589793
• E (or e) — Euler's number ≈ 2.718281828459045
• TAU (or tau) — 2π ≈ 6.283185307179586
• Inf — positive infinity (already exists as a float literal)
• NaN — not-a-number (already exists as a float literal)

Motivation

Came up when writing tests for trig functions (023_trig_functions.ndc) — to test sin(pi/2) == 1.0 we had to inline 1.5707963267948966 as a magic number, which is fragile and unreadable.

Possible implementation

Register as global constants in ndc_stdlib/src/math.rs, similar to how NaN and Inf are likely handled.