feat: add stringAgg aggregate function

.changeset/add-string-agg.md

@@ -0,0 +1,6 @@
+---
+'@tanstack/db-ivm': patch
+'@tanstack/db': patch
+---
+
+Add `stringAgg` aggregate function for concatenating string values within groups. Supports configurable separators and ordering with efficient incremental maintenance via binary search and fast-path text splicing for head/tail changes.

docs/guides/live-queries.md

@@ -1122,7 +1122,7 @@ const userStats = createCollection(liveQueryCollectionOptions({
 Use various aggregate functions to summarize your data:
 
 ```ts
-import { count, sum, avg, min, max } from '@tanstack/db'
+import { count, sum, avg, min, max, stringAgg } from '@tanstack/db'
 
 const orderStats = createCollection(liveQueryCollectionOptions({
   query: (q) =>
@@ -1136,6 +1136,7 @@ const orderStats = createCollection(liveQueryCollectionOptions({
         avgOrderValue: avg(order.amount),
         minOrder: min(order.amount),
         maxOrder: max(order.amount),
+        statusTimeline: stringAgg(order.status, ' -> ', order.createdAt),
       }))
 }))
 ```
@@ -2254,6 +2255,15 @@ min(user.salary)
 max(order.amount)
 ```
 
+#### `stringAgg(value)`, `stringAgg(value, orderBy)`, `stringAgg(value, separator)`, `stringAgg(value, separator, orderBy)`
+Concatenate string values within each group. When `orderBy` is omitted, TanStack DB falls back to the source row key for deterministic ordering:
+```ts
+stringAgg(delta.text)                  // Deterministic fallback order by row key
+stringAgg(delta.text, delta.createdAt) // Ordered by createdAt
+stringAgg(delta.text, ' ')             // Custom separator with fallback order by row key
+stringAgg(delta.text, ' ', delta.seq)  // Ordered by seq with separator
+```
+
 ### Function Composition
 
 Functions can be composed and chained: