chore: changeset for the compensated sum / avg on every face

Claude-Session: https://claude.ai/code/session_01DEvba2nBuD4tWzfq8r8NFY
Co-authored-by: Claude <noreply@anthropic.com>

Author: claude

.changeset/20544-compensated-sum-every-face.md

@@ -0,0 +1,47 @@
+---
+'@objectstack/core': minor
+'@objectstack/objectql': patch
+'@objectstack/driver-memory': patch
+'@objectstack/service-analytics': patch
+---
+
+fix: `sum` / `avg` answer the same double on every face the platform owns, added with one compensated fold that `@objectstack/core` now exports as `compensatedSum` (#20544)
+
+Clause-②: yes
+
+**New export.** `@objectstack/core` exports `compensatedSum(nums)`: the sum of
+`nums`, added in order with Kahan-Babuska-Neumaier compensation, which is the
+summation SQLite (3.43 and later) uses for its own `sum` and `avg`. It moved
+here from `@objectstack/objectql`'s rows path (`in-memory-aggregation.ts`),
+which now imports it instead of keeping a private copy.
+
+**What changed.** Three folds still added a group's values naively, and now call
+the same function:
+
+- `@objectstack/driver-memory`'s `aggregate()` and `find()` with aggregations,
+  the path `engine.aggregate` takes on an in-memory datasource;
+- `@objectstack/driver-memory`'s analytics face (`MemoryAnalyticsService`),
+  whose `sum` / `avg` measures are now a `$group` `$accumulator` in place of
+  mingo's `$sum` / `$avg`;
+- `@objectstack/service-analytics`' draft preview.
+
+Over a `number` column holding `0.1`, `0.2` and `0.3`, each of them answered
+`0.6000000000000001` / `0.20000000000000004`. They now answer `0.6` /
+`0.19999999999999998`, as SQLite and the engine's rows path do. Over
+`1e16, 1, -1e16` they answered `0` and now answer `1`. On driver-memory,
+`engine.aggregate` gave two answers depending on its path: `having { s: { $eq:
+0.6 } }` kept the group on the rows path and dropped it on the native path. It
+now keeps it on both.
+
+**What did not move.** Two addends, integers whose running total stays within
+2^53, and a non-finite total give the same answer as before. Which values count
+as addends did not change either: booleans as 1 / 0, and nulls and non-numeric
+strings left out, as each face already had it. `count`, `min` and `max` are
+untouched. The analytics face's pipeline dump (`result.sql`) now renders the
+accumulator's functions by name, so a `sum` measure and an `avg` measure still
+dump differently.
+
+**Residual.** PostgreSQL and MySQL add their doubles natively without
+compensation, and the platform does not wrap that arithmetic. So over three or
+more fractions their native path can still differ from these faces in the last
+place. An exact `$eq` on a fractional sum compares doubles; compare with a range.