test(gtc): live-WAN integration for GUI segmented downloads (GTC-1/2)

Drives the new GUI segmented helpers
(provider_segmented_download_eligible,
run_provider_segmented_download) end-to-end against the axpbuntu
Hetzner lab, the missing piece to the CLI-only baseline harness
under tests/gtc/. All tests #[ignore] by default and SKIP gracefully
when the vault is locked or the lab profile is missing, so default
cargo test stays offline-safe.

Coverage:

- SFTP @ axpbuntu (SftpConnectionPool, password+key admin profile):
  64 MiB single-stream vs segments=4 byte-identical via sha256, plus
  a cancel-cleanup variant that proves TempFileGuard removes the
  .aerotmp on a pre-cancelled token (mid-flight cancel is
  timing-fragile for 16 MiB windows because the engine's 64 MiB
  inner-read cap collapses the per-window loop to one iteration).

- FTP @ axpbuntu (FtpConnectionPool, seeded vsftpd profile): same
  byte-identity cell, also validates the read_range
  ensure_connected fix that landed in the previous commit.

- S3 / MinIO @ axpbuntu (HttpClonePool, the other transport
  family): same byte-identity cell, proves the GUI helper works
  across all three ProviderTransferExecutorKind variants the
  eligibility gate accepts.

- A purely-local eligibility test that runs without the lab (no
  --ignored) and validates the anti-fragmentation / pool-kind
  gate at the unit boundary.

integration_sftp_pool.rs: three pre-existing TransferSettingsInput
literal callsites missed the new download_segments field added by
GTC-1, breaking cargo test for the whole crate. Added the explicit
download_segments: None at each site so the test file compiles
again. This is not a new test, just a source-compat fix to an
existing one.

Author: axpnet