v0.128.0: chunked browser .fab upload on /import (tunnel-proof)

Cloudflare edge caps request bodies (~100 MB, probed live: 120 MiB -> edge 413,
80 MiB -> origin), so the client slices the file into 64 MiB strictly-sequential
chunks; the server streams each to a .part file in the default drive's exports
dir and finalize renames atomically. Scan/validate/import pipeline untouched.
upload/{init,chunk,finalize,abort} inside ServeExportAPI (inherits auth+CSRF);
single-flight; offset==received or 409+echo; free-space gate; collision ->
lowest-free "name (N).fab"; startup GC of *.part-*; 15-min idle abort.
appexport.DiskFree exported (seam web.uploadDiskFree). Scenarios A-F tested,
red-proofs run (traversal / out-of-order / overwrite).

REUSE.md

@@ -97,6 +97,7 @@
 | `system.IsMountPoint` / `IsWritable` / `PathsOverlap` | controller/internal/system/mounts_linux.go | `(path) bool` | Mount checks | `_other.go` stubs return permissive values — Linux behavior is the real one |
 | `system.CheckBackupDestination` | controller/internal/system/mounts_linux.go | `(path) DestinationHealth` | Tier2/offbox target vetting | Detects same-physical-device (`SamePhysicalDevice`) |
 | `system.ProbeStoragePath` | controller/internal/system/mounts_linux.go | `(path) ProbeResult` | Disconnect detection | — |
+| `appexport.DiskFree` | controller/internal/appexport/estimate.go | `(path) int64` | Free bytes for space gates (df-based, 0 on any error) | Exported v0.128.0 for the browser-upload gate; test seam = `web.uploadDiskFree` package var |
 | `planDriveGates` / `Server.ReconcileDriveGates` | controller/internal/web/intermediary.go | pure plan + executor | Drive appear/disappear reactions | `planDriveGates` is PURE (unit-testable); loop at `driveGateLoop` |
 | `Server.runStorageInit` / `runStorageAttach` | controller/internal/web/storage_handlers.go | wizard pipelines | New-drive enroll / re-attach | Format goes through the agent's two-step confirm (below) |