C6B-F1 cause 1 + §8: additive .fab export (HDD binds AND named volumes)

executeExport no longer either/or gates user data on needs_hdd — a needs_hdd
app bundles BOTH its HDD mounts and its named volumes (sonarr_config = the
whole app DB was silently dropped pre-fix). exportHDDData returns error and
fails LOUDLY on a basename collision between mounts (the manifest keys tars by
basename; the old code silently overwrote the first tar). EstimateExport made
additive to match, so the fits-on-dest gate counts both. Round-trip placement
test proves a userdata tar restores to <HDD_PATH>/userdata through the
untouched import mapping. Red-proofs recorded: either/or revert fails
scenario A; collision-check removal fails the collision test.

controller/internal/appexport/estimate_volsize_test.go

@@ -8,14 +8,17 @@ import (
 	"testing"
 )
 
-// hddProvider is an rtProvider that reports an HDD-backed stack (for the regression scenario H).
+// hddProvider is an rtProvider that reports an HDD-backed stack (estimate scenario H + the
+// v0.130.0 additive-export tests in export_additive_test.go).
 type hddProvider struct {
 	*rtProvider
-	mounts []string
+	mounts  []string
+	hddPath string
 }
 
-func (p *hddProvider) GetStackNeedsHDD(string) bool     { return true }
+func (p *hddProvider) GetStackNeedsHDD(string) bool      { return true }
 func (p *hddProvider) GetStackHDDMounts(string) []string { return p.mounts }
+func (p *hddProvider) GetStackHDDPath(string) string     { return p.hddPath }
 
 func newEstimator(t *testing.T, provider ExportStackProvider) *Exporter {
 	t.Helper()