Function buildPolynomial

src/compute-engine/library/statistics.ts:1196–1215

Build `c0 + c1·v + c2·v² + …` with canonical construction (no simplify). * Terms with an exactly-zero coefficient are skipped so the fitted expression * reads `x² + 1`, not `x² + 0x + 1`.

(
  ce: ComputeEngine,
  coeffs: Expression[],
  variable: string
)

      // passes.
      // The LEADING arm is the string-preservation rule: a sample drawn from a
      // string's own characters is a string (`docs/STRING_ROADMAP.md`, "String
      // preservation rule"; promoted in Phase 2 as an ELEMENT-PRESERVING
      // list-out operator). `RandomSample` is eager and has no lazy collection
      // handlers, so the join happens in the `evaluate` handler below.
      // Re-segmentation caveat: rejoining the sampled characters can merge or
      // split grapheme clusters, so the result may hold a different number of
      // characters than `k` — a sampled combining mark attaches to whichever
      // character now precedes it.
      // Spelled as a BOUNDED type variable (`T where T: string`), never the
      // ground type `string`: an `unknown`- or `any`-typed operand refutes no
      // arm, so a ground `string` parameter would win most-specific-wins on
      // every untyped operand and claim `string` for a call that usually
      // returns a list. A bounded variable with no call-site binding does not.
      signature:
        '((T, number) random -> T where T: string) & ((indexed_collection, number) random -> list)',
      // IMPURE producer: decline-only from the source facet — zero draws,
      // never `true` (see `RandomShuffle`/`RandomChoice`).
      canEnumerate: (expr) => {
        if (!isFunction(expr)) return undefined;
        return expr.op1.isEnumerableCollection === false ? false : undefined;
      },
      evaluate: ([xs, kOp], { engine: ce }) => {
        if (!xs.isIndexedCollection) return undefined;