[feature] : export entropy and collision-probability helpers #37

The docs explain the collision maths, but users can't compute it for their own configuration.
Three pure functions would make it usable, taking the same options and defaults as createId
(size 24, base62):

entropyBits(options?: IdOptions): number
sizeForEntropy(bits: number, alphabet?: string): number
collisionProbability(count: number, options?: IdOptions): number

So createId({ size: sizeForEntropy(128) }) replaces guesswork, and
collisionProbability(1e9) answers "is the default safe for us?"

The trap: floating-point precision

The textbook formula 1 - Math.exp(-n²/2N) returns exactly 0 for realistic inputs, because
Math.exp(-4.8e-26) rounds to 1. Compute it in log space and use Math.expm1:

const x = Math.exp(Math.log(n) + Math.log(n - 1) - Math.LN2 - size * Math.log(alphabetLength));
return -Math.expm1(-x);

Reference values for tests

Acceptance criteria

• New src/utils/entropy.ts, exported from src/index.ts
• The reference values above as tests, plus count < 2 returning 0 and input validation
  (alphabet of at least 2 characters, positive integer size, non-negative count)
• Results agree with the "Uniqueness & Collisions" docs page
• CHANGELOG entry under Unreleased; npm run verify passes; no inline comments in src/