Read the export test's heap only after pools drop their caches (closes #511)

`TestArchiveExport_Streams` read the heap after one garbage collection, but the libraries the export calls (`regexp` under GORM's table names, `encoding/json`, GORM's row scanning) keep spare buffers in a `sync.Pool`, which keeps them through one collection. Each reading therefore counted however many happened to be cached, which varied by about as much as the test's limit and failed `go test` on `next`. The test now collects twice before each reading, so a reading is the memory the export holds. The limit, the row counts and the claim are unchanged, and an export that does not stream still fails it.

Unverified: the branch's CI run waits for the runner outage to clear.

Model: opus-5-5

internal/delivery/target_database_export_test.go

@@ -460,8 +460,13 @@ func TestArchiveExport_OneFileOpenAtATime(t *testing.T) {
 }
 
 // heapPeak is an io.Writer that discards what it is given and records
-// the largest heap it saw at a write. It collects garbage before each
+// the largest heap it saw at a write. It collects garbage twice before
-// reading, so the heap it reads is what is still held.
+// each reading, so the heap it reads is what is still held. Once is not
+// enough: the libraries the export calls (regexp, under GORM's table
+// names, and encoding/json among them) cache buffers in a sync.Pool,
+// which keeps them through one collection, so after one the reading
+// counts however many happen to be cached. That varies from run to run
+// by about as much as the limit in TestArchiveExport_Streams.
 type heapPeak struct {
 	max uint64
 }
@@ -469,6 +474,7 @@ type heapPeak struct {
 func (p *heapPeak) Write(b []byte) (int, error) {
 	var m runtime.MemStats
 
+	runtime.GC()
 	runtime.GC()
 	runtime.ReadMemStats(&m)
 	p.max = max(p.max, m.HeapAlloc)
@@ -498,6 +504,8 @@ func exportHeapGrowth(t *testing.T, rows, bodySize int) uint64 {
 
 	export := listExport(t, path)
 
+	// Twice, for the reason heapPeak gives.
+	runtime.GC()
 	runtime.GC()
 
 	var start runtime.MemStats