refactor(pure): share MS_PER_DAY with tests and document pruneHistory

src/pure.js

@@ -1,5 +1,5 @@
 const SUFFIXES = { k: 1e3, m: 1e6, b: 1e9, t: 1e12 };
-const MS_PER_DAY = 86_400_000;
+export const MS_PER_DAY = 86_400_000;
 
 export function parseTarget(input) {
   if (input === null || input === undefined || input === '') return null;
@@ -53,6 +53,14 @@ export function computeEstimate(current, target, perTrain, perDay) {
 
 const THIRTY_DAYS_MS = 30 * MS_PER_DAY;
 
+/**
+ * Returns a new array of entries from the last 30 days, with the 30-day
+ * boundary treated as exclusive (an entry exactly 30 days old is dropped).
+ * Does not mutate the input.
+ * @param {{ts:number, delta:number}[]} entries
+ * @param {number} [now] Reference time in ms (defaults to Date.now()).
+ * @returns {{ts:number, delta:number}[]}
+ */
 export function pruneHistory(entries, now = Date.now()) {
   const cutoff = now - THIRTY_DAYS_MS;
   return entries.filter((e) => e.ts > cutoff);

tests/pure.test.js

@@ -1,8 +1,8 @@
 import { test } from 'node:test';
 import assert from 'node:assert/strict';
-import { parseTarget, computeEstimate, pruneHistory } from '../src/pure.js';
+import { parseTarget, computeEstimate, pruneHistory, MS_PER_DAY } from '../src/pure.js';
 
-const DAY = 86_400_000;
+const DAY = MS_PER_DAY;
 const NOW = 1_700_000_000_000; // fixed reference
 
 test('parseTarget: integer numbers', () => {